Welcome to Hogwarts, School of Witchcraft and Wizardry.
Over this past summer The Ministry of Magic, in response to the demographic crisis, has instituted a new policy placing all female student eligible for consumption, by either the student body, the faculty, the staff, or the many magical creatures in and about the castle. The staff and faculty generally only eat girls as disciplinary action, but the students are another story. They are in fierce competition to protect the girls from their own house as they try to kidnap girls from other houses, either to eat, or feed to monsters:
Every girl who is captured and devoured legitimately counts for a point for the capturing student's house and against the kidnapped girls house. She also provide much entertainment to that house as they are she in a feast or fed to someone's pet, sometimes after being ravaged by the student that captured her. However, this is not a complete free-for-all. There are a number of rules and restrictions in place to prevent complete chaos - for girls to count towards the point total, they must be captured legally. Moreover, violating these rules also brings strict punishments, as described below:
The rules of this new policy, as well as other aspects of this world, are as follows: (Don't be daunted by the amount of detail - I'm just trying to give you a sense of what kind of world this is. I'm more interested in the spirit of the rule - if you need to have an exception made, just come up with a decent sounding reason).
POPULATION:
At any given time, the population of the school is roughly 50/50 male/female. Since there are so many surplus girls in the general population, new girls get transferred in as others are eaten.
MENU:
Only girls are on the menu. This rule is absolute and enforced by binding magic, and applies even outside of Hogwarts.
SAFETY: Girls do have some limited protections: the following rules are not enforced by magic, but rather by the faculty, staff, and prefects:
a) Students MAY NOT enter other houses unless invited by a student from that house.
b) Girls may not be eaten in their own houses (ie, Gryffindor tower, Ravenclaw Tower, Slytherin Dungeon, Hufflepuff Basement), unless their prefects allow it (usually theu won't, because of the house cup, as well as house loyalty).
c) Girls may not be captured or eaten in the Main halls (which provide access to the classrooms). They also may not be captured or eaten in class, unless their teacher AND the head of their own house (i.e. McGonagall, Snape, Sprout, or Flitwick) sign off on it.
DUELING: Anywhere else in the castle, the rule is that you may capture a girl, but only if you challenge her to a duel first (which she HAS to accept). Any companion of hers within a 15 foot radius may fight in her place to protect her, however. If the challenger is male, he may not make the challenge without a WILLING stand-in offering to be "collateral".
The winner gets the loser (or the girl they were protecting or using for collateral, if they are male), so one way or another, a girl will be captured by someone. At that point ,a binding spell is placed on that girl, and she may be taken away by the victor of the fight, usually to the victor's House where she can be chained up. If someone wants to rescue her (or poach her), and they are still in the castle, then they have to make ANOTHER challenge (and so if they are a guy, they must provide ANOTHER girl, who also must be willing, as collateral. As you can imagine, its hard to convince a second girl to be your collateral when the first one just got taken away to someone/something's dinner).
NO MAN'S LAND: Outside the castle it's a free-for-all, so anyone can try to kidnap girls EXCEPT during organized events, like herbology class, quidditch, etc... Of course, these events have their own dangers...
RULE VIOLATIONS:
If students fail to follow the rules, or start failing classes, or does any other serious offense, and are caught, then the punishment varies depending on their gender:
A girl found in violation is generally sentenced to be eaten in some way - either fed to monsters for entertainment (and to keep them from going hungry), served at a feast, given to the house of the girl she illegally attacked, etcetera.
If a guy breaks the rule, he can be expelled, or possibly have a restraining spell put on him to keep him from eating ANY girls for some amount of time. It may even be decided that a girl from his house, or even a sister or cousin, has to pay the price.
HOWEVER, in all cases, the offending student (male or female) is innocent until proven guilty - so if a 5th year student snatches a little first year girl from the main hallways when no one's looking, or grabs someone elsewhere without making a formal challenge, and doesn't get cuaght, he's fine. As long as no one can prove conclusively that he took a girl illegally, he's fine (remember, he can always say he caught her outside, unless she can prove otherwise.)
CAPTIVE GIRLS:
As long as the above rules are followed, students may do what they want with their captives, which often means rape, followed by (or even simultaneously with) a feast or feeding her to a monster for fun. However, girls can also be used in a prisoner exchange, or held as a slave for a little while. If they aren't eaten before the end of the year, they will go free and not count towards the house cup, so although some girls are kept as slaves for awhile, they are almost always eaten within a few weeks - since they will be eaten anyway, its better to avoid the risk of them escaping (plus each house has only limited space for holding captives)
SEX:
Sex between students is allowed, (after all, life is short). An unbreakable birth control/protection spell has been placed on all the girls, so there is no risk of pregnancy or STDS. Captured girls may be raped.
QUIDDITCH:
The Quidditch teams are required to have at least 3 girls on the team at all times. The winner of each match gets to take their pick of the losing team's girls as a victory feast. There are also sometimes hazards during the game itself - the game can be played over a pit of monsters, or with dragons flying around the air. And of course, the penalty box almost always has a fun surprise.
PREFECTS:
House prefects have considerable authority. Often, they get to decide which girls are sacrificed when their house is punished. Technically they have discretion over the use of all captured girls (with any disputes among prefects resolved by the, though in practice, a girl is usually given to the one who captured her to do as he or she pleases, unless there is a pressing need for her (e.g. in a prisoner exchange), since the prefects like to keep morale up. As mentioned above, they can also put girls in their own houses on the menu as a way to punish them, or their friends, for disloyalty. Students can appeal to the head of the house if this is abused, though such abuse is rare, on account of the prefects' interest in keeping morale up.
THE HOUSE CUP:
The house cup is given to the house that has the highest score, evaluated by taking the number of girls they have eaten from other houses, and subtracting the girls that have been eaten from their own house. This gives them incentive to protect their own girls. The winning house is given a feast, consisting of a number of the losing houses' girls, generally selected from that house, with one exception: the winning prefects each get the privilege of selecting any girl he or she wants from the entire female population of the lowest scoring house. This is often the only practical way to get their hands on a rival prefect (or rival prefect's girlfriend, if the rival is male), and so the prefects tend to be especially fanatical about winning.
COMARADERIE:
In each house, there is also in each house a sense of camaraderie, and the male students tend to feel particularly protective of the girls in their house. While this isn't universal - there are always somewhat disloyal exceptions, the general attitude is that they are all in this together against the other houses, and particularly their rivals. (That DOESN'T mean I want to discourage you from having Harry or Ron eat Hermione, or something like that. Just make sure that you come up with a plausible explanation, e.g., they have a falling out, or whatever - and other people in the same house wouldn't shrug it off - there might be consequences). If you really want, I guess you can put a character in a different house if that makes it easier.
I think that's about it. Like I said, don't feel too confined by the rules - they are guidelines to give you some place to start, so if you feel like you need to deviate from them, just make up a reason. For example, you may have faculty women get eaten also, but just give it a good explanation, like she committed a firing offense, or a monster got loose, or something like that. After all, accidents happen.
Now to start, what year, i.e., which book does the story take place in? Make sure that the ages of characters correspond to their ages in that book.
One idea which you don't have to do, but will make things interesting, is to take plot ideas from the actual books and give them a vore/cannibal twist - Maybe the Chamber of secrets is opened, just like in the books, but rather than turning people to stone, well, let's just say you can probably imagine what the Basilisk is doing in this Interactive.
You may also set the book at the time James, Lilly, Snape, et al are in school. You can even put it in the time when Voldemort was a student if you want.