Peter's age is pretty much fixed. Nicola and Lawrie are 14 and a half in Run Away Home, because Nicola tells Giles so. Ginty is 16 on Twelfth Night. Peter comes between Ginty and Nicola, so he is 15 and a quarter, and turned 15 in September just gone, so he shouldn't be taking O Levels this year anyway. He'll take them in 18 months time, when he's 16 and three quarters.

Nicola and Lawrie would be in the same school year as him, except that they appear to have missed a year too. Probably because of their incessant illnesses up to age 12.

Colne_dsr