Up to 280 people attended the party looking back at 100-years of memories at the Leeds Road seat of learning.
Headteacher Paul Eckersley kicked-off the festivities with an instructional welcome before visitors were taken on a tour of the four exhibitions of photos and memorabilia by students.
In the afternoon, 600 balloons were released and a time capsule – containing a DVD of photos, school tie, prospectus, centenary badge and copy of Selby Times – was buried on the front lawn.
To round off the celebrations, everyone was invited to sing the old school song and a chorus of happy birthday, before the specially decorated cake was distributed with champagne.
Arts director Ann Wallace said: "A lot of people signed the visitors book before they left with information about themselves so we can put people in touch with old friends.
"The vast majority of people said it was a really good day, and they loved seeing all the photos and memorabilia as well as singing the school song."