Your name
Your address

Principal name and Address.

Dear Mr/Mrs______

I am writing this letter to inform you about the misbehaviour of a few students belonging to high school during recess.
They were constantly misbehaving with other students by bullying younger kids
and were causing a lot of trouble to not only them but also to the other kids playing around.

Please take immediate action about this as otherwise it may cause a lot of trouble and disturbance in the school campus during recess and may hurt someone badly.

Sincerely ________ 
But still respect should be given
And the format of formal letter also tells this
The principal of the school is someone who has seen you, knows who you are and cares about your education hence they can be respected as Dear.
Because we are showing respect but at the same time we don't need to say 'Respected' that would we odd.
Imagine saying that to your principal in real life.
But whenever we write letter in exams we should use respected otherwise teacher will cut marks