in much the same way that the ignorant and the mentally challenged insist on calling their local supermarket Tesco's
It is rare that I rise to such bait, but first of all, people calling their local supermarket 'Tesco's' probably do so verbally, so the apostrophe is not spoken but inferred, and secondly, presumably the meaning is 'the supermarket of Tesco', so it is 'Tesco's [supermarket]'. Quite why such disparaging language emanates from you on the point is rather hard to understand. People say it because people say it, that is how language works and it presumably works in their respective heads as sounding right.
Coming back OT, I think that in my time, there might have been 1 or 2 expulsions from CH. There was, AIUI, a rustication of a Dep. (IIRC) after a serious indecent assault by pupil on pupil(s) round about 1980-1981. I can't recall any particular expulsions, more likely those too challenging for the School seemed to leave presumably at an agreed point after discussions with parents and some less academically-gifted left after 'O'-level.
It was not unknown for some older pupils to have notches in the belt to indicate the number of cane strokes received. I remember one bloke who c. 1981 had a belt that looked like a saw blade.