A plurality of exceptions

WITH regard to the school 
which was hauled over the 
coals for misusing an apostrophe (Yorkshire Post, September 15), it is quite possible that 
the composer of the offending notice is only doing as he was taught.

As a pupil in the 1960’s (sic), I was taught that there are certain exceptions where the apostrophe is indeed used to denote a 
plural. MP’s, p’s and q’s, 6’s and 7’s were examples I can remember.