They are not plecos.
They're generally peaceful, have never once heard of them killing another fish, and I'd be incredibly surprised if it happened. Finding a dead body doesn't prove a thing, fish die for all sorts of reasons, and most fish will eat the body of another fish - it doesn't mean they killed it, they're just not turning down a free meal.
We'd need to know a lot more about the tank to stand a chance of helping. But trying to get another fish that will eat them is kinda sick, and a terrible idea of tank management, which makes me doubt your teacher has any real knowledge or skill in aquarium keeping. Any fish large enough to eat them is also going to eat everything else in the tank, you know...
If he doesn't like them or want them, catch them and return or rehome them. Don't sentence them to death for something they almost certainly didn't do.
Without knowing more about the stocking and water conditions, we cannot possibly know what may have killed other fish. We'd need information to be able to offer any real advice.