There's no way the students could have known that their cameras could be or might be operated remotely. Had the school informed them that this could and would be done, they could have covered the camera in some way. They were not informed, and they had no reason to anticipate the need to take such precautions.

And it wasn't what the students were doing with the computer that was being monitored. It was what the students were doing in their homes in the vicinity of the computers that was being monitored. It wasn't the computers that were being monitored. It was the students, via a camera, that were being monitored. There really is a very big difference between these two things. Monitoring the students without their knowledge via a camera placed in their homes, even if it's in a computer that they've brought into their homes themselves, is illegal. If it wasn't, the government would be able to put a camera in your house and use it without your knowledge and without a warrant, to spy on you. The government can't do that legally, and neither can a school system (which, in the case of a public school system, is the government).