The E-mail address is posted in public, on the school's official website. The act of sending E-mail to that address definitely shouldn't be illegal.

The other legal issue one could run into is harassment - Massachusetts law (Chapter 265, Section 43A) defines harassment as "willfully and maliciously [engaging] in a knowing pattern of conduct or series of acts over a period of time directed at a specific person, which seriously alarms that person and would cause a reasonable person to suffer substantial emotional distress"

Interpret that as you will - I could see someone making a whiny brat plea in front of a court, and given the way this idiot is responding to E-mails by forwarding them to the police, I wouldn't put it past him. However, felony harassment is something that's up to the prosecutor's office to indite for, rather than the principal of a high school, so I doubt anything would come of it.

So, yes, it could conceivably get us into trouble, just like conceivably, you could get hit by lightning while roller-skating. I'd give them about equal odds. Just for a bit of protection, you might want to include more in the E-mail than just "meep" - something like
"I protest your ban on the word "meep" and the associated infringement on the students' first amendment rights to free speech. While I understand and agree that disruptions to classes are intolerable, I believe that banning the use of a word is both overreacting and ineffective - what the meep will you do if they start using "the" or "math" instead?"
should lower the odds of legal action to something resembling those of getting hit by lightning while roller-skating indoors.

(disclaimer: I am not a lawyer)

But things like this are just a little bit too ridiculous.