gregbo - 10:10 pm on Nov 7, 2006 (gmt 0)

I can see that a small # of IPs with the same user agent occurring during the same time of day could trigger a fraud alert. This is, unfortunately, one of the dark sides of AdSense: legitimate clickstreams might be flagged as fraudulent if they match clickstreams that were previously identified as fraudulent.

If you actually know the students who are clicking, and you're let back in, perhaps you should explain to them that repeated clicking (without buying or performing some sort of action at the advertiser's site) could get you banned.