You can do the ball bearing + epoxy trick but I don't think you'll like it once it comes to adjustments.
Since you already got the pits for most of the major things, you might as well get the seat post lock, or just make your own out of an old chain+inner tube. You might be even able to ask your LBS for the raw materials. Just need a chain tool to set it to the correct length and setting the pin. If you leave your bike for any extended amount of time, I doubt you can prevent component theft as much as them wanting to simply take your bike.
Since it is for school, ask the teacher(s) if you can bring the bike in with you. If you can't don't ride a nice bike to school. If it's a large lecture hall style, I don't bother asking and just bring my bike in. Usually there's plenty of room.