Don't bother, its a waste of your time, for many reasons.
To write an anonymous letter to your head teacher only diminishes you. If you feel you have something to say then say it, don't hide behind anonimity. Also, you may think the teachers and the head wont know who its from but they always find out, trust me on that one.
Why not do something constructive like form or join a student council so that you may have some input into the areas that you feel are failing due to the head teachers running. Whilst you may feel you have genuine grievences have you ever considered what his agenda is and how he has to make targets etc, sad though it is, schools are not just about teaching these days, everything is scrutinised by the 'inspectors'..
You also say you dislike your head teacher, that is a personality issue, and believe me, there will be many more people in you lifetime you will come across who you dislike, thats because we are all different.
You sound like a pretty smart guy, use you talents constructively not destructively.