Sort of following on from the last point and not trying to be awkward, my favourite "season finale", is Series 7. Granted the musifc definitely ages it, however I think it is quite defining in many ways because I believe it does mark the end of what could possibly be classed as the first era, that runs from 78-84, an era where the show is set in a smaller school, with an apparently smaller cast of characters, and in some respects, bar the third years and Miss McCluskey, effectively a completely new cast as well.
I've said before, but there is episodes or sometimes a couple of episodes in a row when the defintion of the eras is so marked and this is one. It does feel in a way like a dance or disco at the end of term, where you all meet up, knowing that you won't see them again. You actually feel like you have seen many of them grow up and now they're leaving, perhaps how teachers feel at these things as well.
Series 10 should have the feeling, but I don't feel it. Perhaps because the music used was more party, than the Spandau Ballet track used in Series 7. Series 12 has the same feeling in a way to Series 7. I like Mrs McCluskey leaving montage at the end of the episode, however I actually struggle to remember what happens in the rest of the episode which ultimately can't be good.
Series seven is amy all time favourite Grange Hill finale too.