The tool must be automatic and always available. Also, it must not filter out more good sites than bad ones, by locking the users into a specific small set of web sites, for example. It must have automatic blocking features that require little or no administration. It must have a password for the teacher or administrator to disable filtering, but be unbreakable without the password. Price per seat, assuming 50 seats.
Humans are better able to avoid filtering out good sites.
The following products were not considered because they didn't have the basic features described above. Gulliver's Guardian , by Gulliver Software . Cyber Snoop by Pearl Software . Has no filtering features. Only monitoring and tracking.