It happened "in the school grounds", not in the lab. And they aren't telling us what she mixed, or what her intent was. There's more to this than meets the eye.
I am inclined to agree with you, it's more than a ridiculous over reaction to a genuine science experiment IMO.
Conducting a "science experiment" at 0700 on school grounds sounds more to me like showing her friends that she could make something that goes bang !
Even by U.S. standards, isn't 0700 early for the start of the school day ?