There's this kid from school who's really trying to figure out my sexuality...?

You can BLOCK him from your facebook page. Just ask him why he wants to know is he writing a book? doing a survey? Ask him questions about the things he's asking you about. He's being invasive as just say "It's none of your damn business", with a smile on your face (remember the smile). Ask him if he has a problem with his own sexuality because he's too interested in yours. Usually this means people are trying to compensate with being insecure. Trying to manipulate and bully you (that's what this is) is his way of getting confidence. Just flat out say in a CROWD "Does it make you feel better about yourself to nose into my love life"? This puts the attention on him, he has to come with a good reason or look like a jerk in front of a lot of people. Or you could start digging into his love life and see how he likes it.