1 Kid has a gun, the entire staff does, I don't think the kid will win.
There's usually one teacher in the classroom with 20-30 students. They could gang up on her, lock the door, take the gun, and then go on a rampage. It's not as if the whole staff is in one classroom waiting for someone to go crazy.
I've never heard of "smart guns", but if they worked then I still don't know if it would be appropriate to handle guns, much less be a student and know that your teacher carries a gun at all times, around children.
What if the teacher was a rapist/psycho himself? He could kill his entire classroom and make a run for it.
Overall, I don't think the idea would be efficient.