The picture I took is of the stone marked 'Long Stone' on the OS map at SX029521. It's also marked as such on the Magic map, though the associated PDF file appears corrupted when I try to view it.
It is indeed in school grounds, and as much as it pains me to say it, checking in The Romance of the Stones it appears that I did indeed snap the 'Gwallon Menhir'.
Maybe one of the kind Eds will move my entry and photo?
Well spotted sir! You do realise of course that I'll now have to revisit next month to see the real Menear Longstone? :-))