You didn't physically abuse him, you didn't swear at him, you didn't threaten to harm him in any way.

I think that you did exactly the right thing. He got caught doing something bad and was yelled at for it. Sounds perfectly reasonable to me.

If I were in your position I would probably ring the Principal and let them know what was going on and that you had spoken to the kid this morning when you saw him at it. That way if there is any drama the Principal can say that they already know about it - and you get your story in first.

Also, if a Principal knows that something has got to the point where it is happening in front of parents they should probably be doing something about it - that kid is out of control.