We are allowed to contact principals for open jobs, but it is rare for any principal to actually respond or even open an unsolicited email. The best way to get a job is to somehow stand out with a principal. An email is not going to cut it. Phone calls and even showing up in person at a school is the way I have gotten jobs in the past. This also worked for many of my friends.
That being said, I could see where some principals would not appreciate the personal approach. Maybe you could try an email first (with the cover sheet pasted in), a follow-up phone call, and finally a walk-in visit.
Good luck!
