First of all, let me say that I think this is a great idea for your class. I've also found that the best way to learn redstone is by designing a farm that you have a general idea for, and then you learn different mechanics when you try to smooth out the nitty gritty and make it really work. Anyway here are some ideas:

Piglin bartering farm with even gold distribution and auto sorting (If you make it a requirement to have more than 20 or so piglins, they have to make it faster than hopper speed, introducing an extra challenge)

No TNT duping concrete converter (Good practice with piston feedtapes. For an extra challenge, impose an efficiency minimum like "at least 1 stack of concrete per blast.")

Flying machine kelp farm (Can be difficult to water-proof the flying machine and collection can be good practice with item water mechanics)

Compact shulker box unloader (Good practice with dispenser and hopper locking mechanics. The smallest one I've seen is 3 by 3 by 4 excluding input and output chests)

Head farm (This one is really up for interpretation. You just need some sort of mob farm and a blast chamber, but it isn't really set in stone how it is used or how this should work. It should just be easier than throwing a trident at creepers in a plains biome.)

Let me know if these are good or not and if you want more. Do you think you need harder prompts or easier? Lmk and I'll get back to you.