In public school? Never.
At home, yes. Before the 8th grade.
Why not? Despite having some of the world's most restrictive firearms legislation, schools up and down the UK host cadet squadrons, and they keep their firearms - such as
this - on school premises in a steel vault, with ammunition stored separately.
When I was a pup my school was still in possession of the Bren gun it was issued by the War Office, to be used by trained/trusted staff in the event of a Nazi invasion. It was kept as an item of interest, although as far as I'm aware it was still operable, albeit having the firing pin removed, which could've easily been replaced. It was kept in a reinforced glass case next to the school trophy cabinet in our headmaster's office.