My classroom computer at school keeps popping up with a warning about a virus, but the button to "clean file" is shaded and can't be clicked. If the antivirus can't remove the virus, what can?

This itself could be a virus so be careful. I would open up your anti virus program and have it run a deep scan. If you have Malwarebytes anti malware, run this also and good luck!

Thanks! Is Malwarebytes available online for free, or do I need to buy it? I'd really like to clean this up because viruses get passed around the school computers through all the jump drives going in and out of them.

Yes it is free and it works really, really well on these rogue programs. They are becoming increasingly common I am sorry to say, but they wouldn't normally have the button greyed out. Unless that is, they have the 'buy now' button and that still works.

Can you please tell us what kind of anti-virus you are using? Most free anti-virus can detect the viruses but they can't clean it until you upgrade to a paid version. If Malwarebytes won't work then you can try MS Essentials if you are running windows.

This isn't true. A proper free AV program will remove the viruses as well. Such things as AVG will do, and Avast, and Avira. But this is a school computer-which means that first the user won't be able to change the software and second, a school wouldn't be using a free program.

You are correct about this. I didn't consider that the computer was in a school.

About the free anti-virus, I said that based on experience. Well maybe my experience happened long ago that free anti-virus now can clean viruses and not just detect it.