It sounds a touch iffy. I'd check another classroom computer, and see what it says. Assuming the second computer reports the expected anti-virus is installed, check the first one again - I know that when corporate pushes out some anti-virus updates to my work computer, I get a 'no anti-virus installed' for a short period of time. If it's still not listed, cross-check processes - you'll need to find the process name on the second computer, and see if you can find a matching process on the first. If you can't find a matching process name, then I'd really complain to IT.