managed list for site blocking using proxy

[mail9]

Hi all,

Does anybody know of a proxy which allows blocking black-listed sites. 
More importantly: Is there a (frequently updated) black-list available which
can be automaticly incorporated into this proxy? 

This proxy is to be used in a primary school. Client (windoze) filters (like
We-Blocker) are an option, but a proxy would be nice. And maybe a first step
towards cheaper software solutions allover, but that'll be a long run.

Cheers,
Jeroen

--
If packman had affected us, we'd all be running around in dark rooms,
munching pills and listening to repetetive music.