What should i write in my persuasive letter about that schools should monitor students social media?

Well first you must decide if you do or do not agree with schools monitoring the social media of its students. Should you agree, you must think of points to argue in persuading someone (presumably a school administrator or parents) that this is the best course of action for safety (cyber bullying, harassment, online predators) purposes and to ensure that education is the focus of your time on the computers in school as opposed to just for entertainment. If you disagree then you should make arguments for persuading against it because of its infringement on the right of privacy.