I honestly don't see what's wrong with doing it at school (mind you this is possibly an unusual view for the UK) - I was taught to use a metalworking lathe and how to handle concentrated sulfuric acid at a (very modest) high school and I know far more people who handle guns regularly than do either of those.
Was marksmanship ever part of the mandatory curriculum? I didn't think so even back then. Marksmanship as an extra-curricular activity was probably more common 50 years ago, but from what I can Google, some high schools still have marksmanship clubs (usually under the guise of the JROTC).
The biggest issue I can think of with shooting ranges honestly is not "violence" or danger of accidents, but the problem of lead exposure -- a problem that people were a bit less aware of back in "older times", and a particular problem for developing children. Given some reports I see (https://www.ncbi.nlm.nih.gov/pubmed/9117191) I personally would not be comfortable with a school shooting range that doesn't try to mitigate this risk.
