I feel like there's something more going on with the old childhood friend - she simply fainted from the touch, which may be embarrassing sure, but not enough to warrant her moving away. I wonder if she got a leveling high and immediately challenged the dungeon, and simply never came back to school (sorta like the sensei here)... or something along those lines.
Yeah, she probably left because she transferred to some elite hero school from the absurd EXP boost.
Sensei was high level and a mere touch had her low diff solo the room she lost her arm to with a full squad behind her.
A middle school girl getting a similar EXP boost would probably be seen as a prodigy. High schools focused combat would be scouting like the second coming of LeBron or Pelé or Gretzky .
Of course this is a story with not much thinking involved and I'm sure she will show up later in the story with a transformation like you have said but...
Try thinking about it for a bit. Imagine you wanted to be the greatest warrior ever (as most girls clearly do). What's the most efficient way you could level up if it was your only goal in life? If you were willing to do anything to absorb more EXP than any other girl, assuming MC is a never-seen-before anomaly?
I'm not sure what you can think of, maybe recruiting a very large group of men for a bukkake or hiring a healer so the men can carry on while you are passed out. Whatever method, I expect it would be quite indecent.
There are probably top adventurers who are well known to have chosen such a method.
Does a high level carry a stigma? Do normal people think to themselves about how the top adventurers got their level?
Now imagine you live in this world. Your daughter, who has always been a shy and quiet girl, has just moved from elementary to middle school when she's taken to hospital. You are told that your daughter has a higher level than many top adventurers with years of experience and the authorities have some questions for you about this. What would you be thinking? What would happen next?
It's hard to imagine a situation where a girl doesn't move school after that kind of level boost.