I hate whitelists. They irritate me no end in educational settings. Of course, they irritate me anywhere, but businesses can do what they want with their networks. In educational settings, though, where a primary goal is (or certainly should be) to teach students to fully exploit the resources of the World Wide Web, the idea of a whitelist seems utterly antithetical to achieving that goal.
Whitelists, of course, imply that all web content is blocked except for a list of approved sites. Blacklists, on the other hand, imply that only websites deemed offensive, inappropriate, or meeting some other criteria are filtered. Most blacklists are populated through some category-driven subscription service.
Whitelists, obviously, are far more restrictive than blacklists. Content filtering is not only a legal obligation that most of us have, but also a reasonable way to enforce acceptable use policies (or at least those pieces related to Internet traffic and content). However, there are no whitelists in the real world. Students will get access to everything; it's our job to ensure that they know how to deal with it.
Interestingly, I just got a sales call from a company called CiPA Filter, an obvious nod to those legal obligations that at least US public schools and libraries have through the Children's Internet Protection Act. CiPA Filter dispenses with whitelists and blacklists entirely, instead searching for "anything good" on a web page, enabling context-sensitive filtering.
Thus, students won't be blocked from searching for chicken breast recipes or sex education sites. I haven't reviewed their product yet, but it points to an important trend: Allow students to search thoroughly, freely, and effectively, leveraging the vast knowledge stores of the Internet, but still isolate them from sites without merit.
My point, regardless of the technical solution you use, is that protecting students (as well as meeting your legal obligations) does not mean blocking wide swaths of the Web. It means providing resources to block the worst of the worst and the least educational of the uneducational, but more importantly, to provide students with the skills to know the difference.