LAMBTON High's official date of birth may be unclear, but that won't stop it from hosting a 40th anniversary celebration this weekend.

Principal Martin Rosser said it was "very mysterious" the school did not have a commemorative plaque affixed to its buildings or hold any records from 1974, but had picked tomorrow to mark the milestone by opening its doors from 11am.

Mr Rosser said the celebrations would include classroom tours, musical performances, trivia, cake cutting, a tree planting and the burial of a time capsule that will include a list comparing prices of basic goods between 1974 and 2014, a school uniform, surveys about student interests, a mobile phone and a bottle of wine.

The date was chosen to ensure the participation of retiring original staff member Shirley Thomson, 80, as well as the class of 2014, who will mark their last day on September 19.

"We see 40 years as a real milestone for us in the school," Mr Rosser said.

"Lambton has always been a highly respected co-educational high school and we maintain that reputation today - we receive up to 200 out of zone applications for year 7 each year."

The school of 1083 students and about 90 staff was opened following the merger of Hamilton Girls' High School and Newcastle Junior Boys' High School and heralded a swing against single sex schools.

Its alumni have included Newcastle MP Sharon Claydon, The Block's Lara Welham,The Biggest Loser's Michelle Bridges, The Bachelor's Tim Robards, Newcastle Jet Ben Kantarovski and 2014 Commonwealth Games cross country mountain biker Cameron Ivory.

Mr Rosser was appointed principal in 2008, the same year the school hosted then-prime minister Kevin Rudd at a community cabinet meeting.