There never were missile or air-deliverable 20MT bombs. For a 1 MT blast, at 10 miles you would only see 1 psi of overpressure -- enough to blow out windows, but not enough to collapse a well-constructed concrete structure like most public school buildings built during the 50's and 60's. See Tutorial on the Effects of Nuclear Weapons pdf, page 6
Keep reading down the thread.
Russians had 20 MT warheads, though, not in the 60s.
By that time, both sides easily had warheads in the range of 5MT to 10MT.
Even pre-MIRVs, independent missiles would have been targeting a city like NYC in multiple groups to ensure the target was obliterated.
The scenario at 12 miles was not unthinkable.
1MT was reached early on in the hydrogen game and surpassed quickly.
Oh, one other thing—at the time,most of us were in schools built of brick and wood frame constructed in the twenties and thirties.
Most schools built in the late fifties and sixties were modern in design and had a lot of aluminum frame window walls in keeping with contemporary architecture, with interior cinder block walls. Not the same protection as a poured concrete building.