Filters are necessary in schools as part of our 'duty of care' to students. However, 'common sense' use together with whitelists allows a school to work well. After all, not particularly strict url filters will simply block any content from Essex, Sussex and Middlesex LEAs (Educational Authorities) - and that's before we even mention Scunthorpe. The guidance for UK schools can be found at
http://safety.ngfl.gov.uk/schools/As with all things like this a balance is needed. With control of such filters being wrestled away from schools this is a major issue. Filtering needs to be different for sixth form students (16+) as compared with primary school students (under 11s). With some county school internet providers a 'one size fits all' solution is the only option. This just doesn't work. My online lesson about the Rise of Hitler was serverely limited a few years ago with the word Hitler being filtered.
At my school we keep control of the filtering and are able to offer different levels of filtering to different age groups. It would seem appropriate to educate students in the dangers of the internet and hence avoid all filtering. Unfortunately this simply cannot happen in schools. With these sick *astards who run sites with similarly misspelt urls to common children's educational sites and suchlike some limited enforced protection is essential.
Filtering does not work if left along - but with combined use of a whitelist for 'allowed' sites, where teachers can request sites to be permitted and so on, a workable balance can be reached.