Recommendations for filtering?

[Charles Austin]

I just received a call from the school principal because a student
ended up on a graphic porn site after a poorly formed google search.
I need to provide some sort of internet filtering as soon as possible,
or the children will be barred from the computer room permanently.

I have seen several recommendations in the past.  I am looking for
something easily implemented, and low maintenance.  I am not concerned
about "false positives", so the solution does not have to be finely
tuned.

Thanks in advance,
Charles