K9 has worked very well for my family over the years, even on underpowered systems where other solutions crashed and burned. It's not designed to be a spy tool like some products, but instead allows a parent to walk away from a family computer with peace of mind that most inappropriate content, unsafe search engines, and certain site categories are blocked. And, if you think a blocked site is appropriate for school, you can enable it temporarily with the admin password. If there's any question of trust in your mind, you can still poke around and see what sites were browsed even if browser history was cleared.
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