Whew - I've now read all 7 pages of this thread! What have I learnt? - That co-ed grammar schools were not the norm. In our small town there was only room for one Grammar School and one Secondary Modern, so they had to be co-ed.
The Secondary Modern was bigger and much better equipped than our Grammar School, so what many have said about the Secondary schools not teaching domestic science so well, wasn't true for us. I envied the Secondary Modern girls; they learnt to type, and not only did they do Home Economics, they actually did proper Economics.
Whilst I was no good at games, gymnastics or athletics, I did see the point of them, and having played hockey has always given me a good enough idea of what's going on when watching a football match, which might be considered useful.
So, to answer the OP - very little that I learnt at school has been useless in later life.