Continuity mistake: During the memorial ceremony for Danny, Ronnie is reading a tribute but starts crying and is unable to finish it. Helen consoles Ronnie by embracing her, the angle changes to Ziggy and Robbie planting the tree and Ronnie and Helen have changed position - both are facing forwards, watching Robbie and Ziggy.
Factual error: On the coach when the camera is on Miss Booth you can see out the window a road sign saying 5 miles to Hatfield and 15 miles to Stevenage. Grange Hill school is set in North London and they are on their way to the field trip on the Isle of Wight via Portsmouth ferry terminal - the coach is heading the wrong way as both these towns are in Hertfordshire, which is north of London, and Portsmouth is on the south coast of England.
Continuity mistake: When Fiona and Calley are watching Ronnie on her new moped she does a right turn, passing a grey BMW waiting to turn right in the opposite direction. The angle changes and the BMW has disappeared as Ronnie rides over the same part of the road again. Also more parked cars have appeared on the road behind her.
Plot hole: When Danny Kendall first appears in series 9 (1986) he is in the third year, then the fourth year in series 10 (1987) but in series 11 (1988) he is put down a year to help him catch up with work he missed as he was in hospital for most of his fourth year. But in series 12 (1989) he is in the fifth year when he would be aged sixteen going on seventeen and he would not be legally obliged to stay on at school, as sixteen was the legal school leaving age then. When Danny died in series 12 the school planted a tree in his memory with a plaque in front of it giving his birth date as 8th January 1973 and death date as 22nd January 1989. His birth date should have been 8th January 1972, as he would have been seventeen years old when he died.