A roof ventilation unit. Rooms had outlets in ceilings, all ducted together to such an outlet. Works on the venturi effect, draw air out, it gets replaced by air coming in through doors and windows, in the good old days when we did not have draught stripping, which keeps all the radon gas in. I will leave the place where this is sited to someone else (I am fairly certain that I know which one this is, but hey?).
dd is spot on; seems treeve and hh knew too ::6: Chill, the pavilion you spoke of was demolished in about 1980 and new classrooms built over the spot. Thanks for the info on this photo treeve.