Hi, welcome.
Sheesh. What a blow.
I know what you mean about how she's normally so fun, and then she becomes aggressive. A lot of us go through those changes ... day to day, or from moment to moment, with-our kids.
The school cannot hospitalize your child. Obviously, they see something wrong but you can certainly have her tested with-o having to admit her. Besides, if she's not suicidal or otherwise an immediate threat, most Phosps won't take her, due to ins. regulations, anyway.
Make some appts. with-neuropsychiatrists and child psychologists and find out what's going on with-your daughter. Sounds like you'll need therapy to deal with-her aggressive behavior, which, at her age, is probably (I'm saying this with-o a diagnosis) due to frustation and impulsivlity and not being able to express herself properly.
Good luck!