Josh and Maddie finished school in Series 7, like a Finn and everyone else in that year, as they were in Year 13 but were in school in Series 8 for some reason.
Also why do Bolton and Paul and them finish school a year early for some reason
Josh didn't complete his exams because he had the episode of schizophrenia. My guess is he had to complete the term and retake exams which is why he shows up in the next season.