Only 3 antiviruses are actually decent for OS X, which means, do not cause a significant slowdown (means, they're not Windows-ish) and work well. The commercial ones are Intego VirusBarrier and Sophos Antivirus. Both very light and unintrusive, you can't even notice they're there. Currently I'm using Sophos.
VirusBarrier is better for single users, Sophos is specialized in Enterprises/Small businesses.
VirusBarrier detects some Windows malware, Sophos contains the full Windows and Mac malware database.
The third option is the only (decent) free one: iAntivirus. It detects Mac-only malware, it's light and fast. The only downside is that some users are reporting CPU problems (CPU usage goes 100% sometimes, randomly, you can read more on their forum).
There's also ClamXAv, but I wouldn't recommend it as it's mainly only for Windows malware, and that to get real time protection you must install an extra component which is actually pretty slow.
The other Avs I've tried (Norton, McAfee) are garbage. Oh, there's also Dr.Web for the Mac, though I haven't tried it.
That's also I've got to say, make your choice.
[Disclaimer: I do not want to promote the usage of anti-virus software under Mac OS X, as it is mainly unneeded, but if your school requires it...]