I like the idea, but a lot of people resent being made to create accounts on random services. We know that Lemmy is good because we’re here, but it might put them off the idea.
Depending on the age of your students, you might have to be careful about the fact that NSFW content is freely available here, and check the terms of your instance to see if they’re allowed to make accounts in the first place e.g. on most social media and forums you have to be 13 or over.