When I was her age I and a small group of friends used to meet at a Cafe near the school for breakfast sometimes an hour before classes started.
Also, for a while I used to visit a friend before school for half an hour to listen to music because my then-crush used to drop by there sometimes too.
That's two reasons I can come up with why she could have left "too early". Just to meet friends (maybe not from her school) or to meet a love interest. Why that person, or persons, have not said anything by now is however very suspicious.