Mrs Aine O'Brien is the former Headteacher of Fowlmere Manor and Co-Headteacher of Elmsmere Manor. She only appears in Series 3 and so from then she is never seen or mentioned again. Apparently, she has put a lot of work into making Fowlmere a great school, despite it being labled the worst school in the country. Though she may be strict, she is really one of the more nicer teachers throughout the Series. She and Crispin Bell absouletely hate each other, and is shown when she gives him a tiny-cupboard like office to work in. Like Mr Byron, she doesn't get a line in a rap at all, but is featured and mentioned in them. She gets mad at Mr Nunn after he doesn't let girls participate in the Boxing Club. She gets fired at the end of Series 3.
- Mrs O'Brien is shown to have a strong rivalry with Crispin Bell. In Cross Country, when they announce the Elmsmere status, they constantly are seen to interrupt each other in the speech, and also Mrs O'Brien encourages Dexter to choose a Fowlmere pupil as the winner of the anthem competition as it will make her look good to the governors when they pick a new head.
- Mrs O'Brien seems to enjoy horse-riding, as in Merger a picture of her and a horse is shown in her office.
- She is married.
- She is more popular than Mr Bell.
- Miss Parkwood refers to her as Uh O'Bren.
- Whilst she might be a low antagonist in Series 3, she is the reason why Josh managers to get into Hillview Academy and become a rapper.