I don't think it was unethical. I think it was brilliant.

First and foremost, I'm tired of the ubiquity of cell phones. I'm tired of seeing them at work, I'm tired of seeing people driving and talking with them, I'm tired of people pulling them out in movie theaters. I'd be really ticked off if I was back in school and people had cell phones ringing during lessons, tests, etc. I get annoyed when people have them at work. It's fine if you are expecting some kind of important call, but too many people in my office indulge in personal calls all day and night and I think it's highly unprofessional (as well as detrimental to their work flow!)

Secondly, I applaud the efforts to get rid of drugs in the school system. I don't think it could be considered entrapment at all since the principal didn't send out messages with any sort of direct (or even ambiguous) requests.

Was it ethical?

How could it not be ethical? The principal is trying to keep drugs out of the school! When given an opportunity like she was handed, HELL YES IT WAS ETHICAL!