Throwing the book at him is a bit harsh... but it really depends on the context. Fair enough, they don't know what he did. Fair enough he copped to the crime and that at first glance appears to be the extent of the "damage".
If this was a one off, okay, laugh it off. "You got me, you little bugger, but I'll get you back." No police needed. In which case this punishment is off the charts heavy handed... the punishment should definitely fit the crime.
If this is the actions of a kid that was constantly engaging in persecution of this teacher, he's been warned, sent to the principal's office, suspended etc. then it's easy to see how it could escalate. I fail to see how anything beyond expulsion is necessary... but some people do tend to blow things out of all proportion.