I used to watch Grange Hill regularly and I also saw the repeats of the 80s series on Sunday mornings. Something I noticed about 90s Grange Hill is that continuity was poor, somethines characters would disappear after a series and would never be mentioned again. This even happened with two main characters, there was a girl called Anna who was a main character in 1995 and 1996, she was seen kissing Chris (Ben Freeman, who was later in Emmerdale) in her last appearance. When the next series started the actress had left and Anna wasn't mentioned once.
The character of Chris, again a main character, disappeared after two series later and was never mentioned again once. There would also be characters who would leave after one year of the sixth from and never be mentioned again.
I also remember Scottish woodwork teacher Mr Mackenzie who was in the series for longer than Bullet Baxter. By 1992, Mr Mackenzie's appearances became less frequent and in the 1993 series he was reduced to a minor character in a few episodes. The actor mentioned in recent years that "the accountants came in and I had fewer scenes", but by the next series Mr Mackenzie had typically just disappeared with no mention.
The only other series I can think of where characters just disappeared on regular occasions was Crossroads. I can't understand why Grange Hill writers didn't mention characters leaving, especially Mr Mackenzie after 9 series.