Intego and Sophos both make good anti-virus packages for the Mac. I believe that both offer academic discounts as well. Check with your school's store; even if they don't offer it directly they may be able to tell you whether these programs are acceptable.
If you are still using OS9, you may be able to get your hands on Norton Anti-Virus or Virex; both products are excellent on that platform. However, avoid their OSX versions, both of which are infamous for creating more problems than they solve.