I agree with Philosopher Clegg that there has to be a degree of consistency with regards character's backgrounds (although a few mistakes are understandable given the amount of characters in it and the number of years it has been on TV). The only major problem I can see is with Seymour's appearance in the First Of The Summer Wine series: that was clearly a sloppy blunder on behalf of the production team. However, as I don't count the prequel as canon, I simply get round it by pretending it never happened. Lol
His knowledge of them in the main series is easily explained by the fact that, although he never went to the same secondary as them, he did go to the same primary school. When Wesley re-introduced them he would, naturally, have had no idea who they were after such a long period of time. The memories, however, would have soon flooded back as he got to know them more.
Remember, also, that Seymour is presented to us as a former headmaster of his own private school and a person who believed himself to belong to a higher social circle. Dissociating himself from his earlier - perhaps lower - life and friends would have been a natural part of his own personal evolution.