How do I tell my classmates about my PTSD?

I agree with telling your teachers, as well as also telling the office secretary, principal, school counselor, and any other staff like that who will need to be understanding of just what it is you might be dealing with if something ever happens in a classroom, or a hallway, or even the cafeteria during lunch. Most of the ones I have ever come across, either when I was a student myself, or later in life when I was working as a custodian have been really good at handling things appropriately, especially the more they know about you and your unique situation.

I'd suggest telling your teachers as well as your classmates personally.