Affiliate Marketing Secrets

I need to write a persuasive essay to my principal as to why I should be allowed to take a summer credit that is not affiliated with my school. When I first talked to my teacher about it he told me that I needed to convince him as to why he should let me go. The school I go to offers to the class I want to take but I would rather take it with this other program. My principal believes that I am doing this because it may be easier, which is not exactly the case.
If anyone could please help me start this it would be greatly appreciated! 🙂