The long and the short of this is that yes they could see what your looking at and you shouldn't expect privacy on a communaly used network that must be administered by someone.
Of course if your not looking at anything like kiddy porn or plans for chemical weapons the fact is that nobody cares what you look at unless it violates the schools fair use codes.
The fact is that no one student is important enough to bother to watch unless activity is noticed that piques an interest.
So the result is that unless your violating a law or school policy no one will care how much porn you download.
