Looking for saying for overreacted with impact

Is "jumped on his pony" a saying?

Im really looking for something with impact
I.e. He overreacted like a .........!
I don't think so. I've heard "jump the gun", but that means to act hastily without thinking it over carefully.

He made a mountain out of a molehill. to act as though a problem is bigger than it really is
You can also play up a problem. / You can overdo something.

It's difficult to help you without some context.

Like the following:
After reading the student's letter the teacher was quick to react emotionally to the accusation.
He was quick to overreact.