( This storyline has a bit different reason why the girl gets superpowers, but it has been approved by scriptboy)
Mary Robertson was a 14 year old girl, who was 5'4", 130 lbs., and had C-Cup breasts. She was also a bit of a nerd. Her life had been completely normal, but that was going to change today...
Mary was helping her mother set up dinner when they both heard a rumbling outside. They lived in a newly built subdivision, but most of the houses were empty, so they didn't have any neighbors yet.
Mary's dad came into the room, curious to what was going on. Mary looked outside the window, and saw...
"It looks like... aliens!" she screamed.
The UFO outside had just beamed down two aliens. Some how the door opened on its own. Mary screamed, and the aliens looked towards her, and aimed a ray gun at her. A green beam came out and hit Mary in the chest.
Her parents thought for a second that Mary's breasts actually grew a little bit, but then dismissed the thought. After being under the effects of the ray for a minute or so, (which appeared to do nothing but make her breasts grow a little bit), Mary passed out.
The aliens then turned to her parents, and started to speak.
"You may wonder what we just did to your daughter." - the both nodded -
"Well, we come from a dimension where humans grow at an astonishing rate, about a foot everyday, and gain tons of muscle in the process. They only start growing that fast a few weeks from when they turn 14. When they do, the boys' dicks become irresistible to women, at least to women in your dimension, and girls' breasts become irresistible to men in your dimension. Also, every single day, after they grow a foot, they receive a new superpower, superstrength not included. This goes on for days, until there are no more superpowers for them to get. Instead, they just keep growing until the age of 20. Now, you're probably wondering why I said so much about humans in our dimension and nothing about your daughter, right?" - again, they both nodded -
"Well, my race is trying to figure out exactly how they grow so much. So we developed a ray gun that can cause people to undergo these changes, and we just used that on your daughter. Well, goodbye!"
And with that, the aliens disappeared. Mary's parents rushed to her, deciding it was best to wait to tell Mary about what will happen to her. Unfortunately, her 14th birthday was a few weeks ago...
Mary woke up the next day with no recollection of what happened the previous day. She got ready to go to school, and then went.