I'm planning to watch through the bedroom window of an attractive classmate who is not my girlfriend.
That is a crime - why would you want to do that?
I want to watch her having sex with someone else even if it's illegal.
That is incredibly inappropriate and intrusive behavior. You shouldn't spy on people without their knowledge, and you certainly shouldn't watch them having sex with someone else without their consent. You should talk to a counselor or therapist about why you feel compelled to do this.
Maybe I don't have to watch her having sex but I would like to watch her masturbate.
Any of those activities is creepy, not to mention illegal. Is this really worth the risk of going to jail for you? You should find better things to do with your time.