Plus, laws differ depending on your jurisdiction.
When I was a kid, we had cassette tapes. We could easily copy music from the radio, and give it to a teacher to play in assembly. It still did not mean that when it came time to review music licensing, the school could avoid paying for doing so.
In fact, copyright licensing of music in schools (especially hymns, for some reason) is one of the most draconian applications of law that I've seen. But that doesn't forgo the schools obligations to properly license it - back then, or now. And, yes, schools that I work in have been threatened with lawsuits for doing things like playing a brief excerpt of a hit single of the day in an end-of-term assembly.
I refuse to allow a school I work in to be sued for something so petty, and be forced to pay over money to record companies (or photographers, or software programmers, or whatever) that's better spent educating children. Applying extra filters costs no more than applying legally-required filters anyway. Licensing of school software is one of the easiest things to fall foul of.... oh, this is a free download, can we just install it everywhere? No. Oh, this is a CD I got in my cornflakes that I'd like the kids to all use? Tough. Licensing says no.
Schools get sued for this. I'm not making it up. It's not *my* pettiness. Hell, some of the largest companies are getting sued for using a photograph from the Internet without permission in their advertising etc. - this is a symptom of people NOT being taught about copyright law when they are in school, and being taught that Google Images is "free". Of course you can find properly-licensed images with Google Images. What you can't do is enforce that setting side-wide through a URL-modifying filter (like you can with SafeSearch).
I'm stopping your kid's school getting sued for 10 x damages for wilfull infringement of copyright and having to pay thousands that would be better spent on, say, computers or properly licensed software or books or teachers, than being paid to some random guy on Tumblr who's had his image pinched and gets bitchy about it.