As a legal matter, it is very likely that a good case that you could make a fair use case for the use of a copyrighted image in an arbitrary school presentation. But unless you're going to teach every student copyright law and make every teacher a copyright expert, it is likely that someone will find a way to use a copyrighted image in a school presentation in a way that wouldn't be fair use. It is much easier to simply say "no copyrighted images" rather than trying to adjudicate fair use on a case-by-case basis.
Practically, it is incredibly unlikely that anyone would sue over a middle/ high schooler's use of copyrighted images. Also practically, if the owner of an image decided to sue, you/ your school/ your district would fold like a cheap suit even if you had an incredibly strong fair use case. The folks that own Star Wars/ Star Trek/ Hogwarts/ etc. intellectual property have teams of lawyers that do copyright enforcement. Adding one more suit has very low marginal cost for them. You/ your school/ your district have 0 lawyers that deal with intellectual property law. Defending such a suit would be very expensive for you even if you won. So practically, it doesn't matter what the legal outcome would be.
Pedagogically, it makes sense to say not to use copyrighted images because teachers are preparing you for the real world. And in the real world, you're not going to want to include copyrighted images in your presentations. In addition to losing the benefit of being educational, it is vastly more likely that some IP owner is going to sue Major Corporate Employer for misusing images in a PowerPoint presentation than that they're going to sue a middle schooler for doing the same thing because Corporate Employer can pay damages. Rather than trying to explain to a bunch of high schoolers that it's probably fine for them to include a comic strip in their presentation but it's not fine for them to do the same thing next year when they're working, they set a "no copyrighted images" rule so you don't develop bad habits in the first place.