Make negatives specifically for the course:

Ask yourself what you want to actually teach in your class, and come up with negatives to function as aids to those lessons.

• If all the students are going to learn is how to mix chemistry, expose photo paper, and what order/timing to dunk the sheets in the tanks, then why on earth is anyone Paying for that when it is basically written on the labels of the stuff they would need to buy to do it themselves?

So what could useful negatives be?

• Focus on technical and primary lesson details. Possibly some simple 'still life' image that carries a variety of textures and details to be explored. Students are there to learn to print, not to walk away with a million dollar masterpiece.
• Light and frame the scene a few different ways, and take lots of duplicates so that everyone can have a copy at hand. [And so that you have spares already made for if a student badly scratches a negative and you decide to still put the course on again.] Bonus points to you if you can come up with a scene that shows off different competing composition options, such as one framing heavily reliant on rule of thirds, while another framing skips that and relies on leading lines instead.
• Design your images to demonstrate "Good and bad" negatives, giving you the opportunity to highlight what makes a negative easy to print, various pitfalls that make it harder, and how basic dodging and burning can recover otherwise 'unusable' images.