Ever wondered why all those around John are laughing? And in particular, ever further wondered why there is a gap in the row immediately behind? Me neither. But here's why...
By all accounts, John viewed his time at school as little more than a never-ending series of opportunities for "ersing aboot". Japery, pranks and casual violence was the order of the day, but most of this "dicking" is, inevitably, lost in the etc. However, this famous school pic actually captures an example thereof.
The missing party behind John is fellow pupil Harry Gooseman. John, apparently, persuaded him to try to "get on the photo twice." I'm not sure about this, but the full, uncropped version is very long, and may actually be a composite of several pics - in which case the idea was to get your photo taken in one location, then sprint along a bit and also appear somewhere else. When the composite was put together, there would be two of you in different places.
Aaaanyway, Harry went too soon, thus not appearing in the first location, but is captured, just, popping up again further along:
Wot larks! Actually, quite amusing and a typically Lennon-esque bit of schoolboy, as I say, "ersing".