Filtering bypass

I work for a large public school district. We have a problem with students and teachers bypassing our web filtering (websense). They are simply pointing their browsers to an outside proxy server and they have unfiltered access. I cannot use group policys as they are older 9x machines and personal laptops. We do not have a network registration system in place and there is no budget for one. Does anyone know how I can prevent this? I am running websense v. 5.5 and cisco PIX version 7. Any help would greatly be appreciated.

Thanks

Chris