Set in an urban secondary school, Big School is a comedy about a dysfunctional staff room, unrequited love and interactive white boards. Chemistry teacher Mr. Church is hopelessly in love with the school's new French teacher, who in turn is being chased by a lothario gym teacher. David Walliams is 40-something chemistry teacher Mr Church, who has taught at Greybridge School for years. Catherine Tate plays the school's enthusiastic new recruit, Miss Postern, whilst Philip Glenister plays the lothario gym teacher Mr Gunn.
It may be start of a new year at Greybridge School, but the teaching staff is as dysfunctional as ever. Miss Postern and Mr Church's relationship has taken a downward turn after a misunderstanding over a local Bella Italia, the woefully underqualified Mr Gunn is now teaching Geography and Mr Barber has a new job as the school caretaker.
When Miss Postern sets up a careers workshop, it seems her slogan 'Which Way Now?' applies as much to her as it does her pupils, not least because the guest speaker is her old teacher training colleague (and now bestselling children's author) Fenella Forbes. But at least things are looking up for one member of staff, as it seems Mr Martin is about to launch a successful pop career.
It's Miss Postern's birthday and she is upset when everyone forgets. But the arrival of a new teacher changes everything and puts her day into a different light. Mr Church and Mr Gunn hear this news and are not happy about it.
It's parents' evening and all the staff want to make sure the event is over before the new episode of 'Great British Bake Off'. Mr Martin and Mr Church spread the gossip that Mr Gunn is the father of a child at the school, as he sets out to make the child's school days as fun as possible. Miss Postern struggles to stop people looking at her cleavage. School Boss (The Head teacher) Ms Baron tries avoiding the event all together.
When Ofsted arrive to conduct an impromptu inspection, Greybridge School is thrown into even more chaos than usual.
The staff celebrate when Mr Hubble chalks up 40 years of service at Greybridge School - although no sooner has he accepted his gift, he drops dead without anyone realising. When Mr Gunn and Mr Church finally cotton on, it's left to the bumbling duo to get the body out of the school before anyone else notices. Meanwhile, Miss Postern has a new poster boy for her anti-bullying campaign - a disabled boy who was moved from his last school because of intimidation. But the well-meaning French teacher soon discovers nothing is as it seems.
A homeless Mr Barber spends the night in the school, only for disaster to strike when he causes a fire in Miss Postern's classroom. It falls on the reluctant Mr Gunn to put a roof over his head. The disaster is good news for Mr Church, as Miss Postern moves into his laboratory while her room is redecorated - but they soon find sharing doesn't come naturally.