I don't know enough about the second-hand market for film cameras, but I respectfully disagree that it's better to use a film camera to teach photography unless your student has specifically requested this. I think digital is a much better format for learning photography for several reasons:
- You can review each shot with your student immediately and offer advice on exposure and composition
- The EXIF data is recorded for each shot, so you don't have to write down the details
- You can change ISO settings between shots instead of having to change the film - this would be a good way to teach exposure
- Digital doesn't cost £4 per roll, plus developing costs. You need to give your student the freedom to take bad shots without worrying about the cost, because each bad shot is a learning opportunity. Also, high speed bursts during action or wildlife photography will become very expensive on film.
- It's time consuming to back up your negatives (i.e. scanning)
- Cheap film cameras might not offer the same range of focus points, focus tracking, exposure modes and other features that a cheap dSLR would provide.