A short ceremony to mark the first day saw pupils from Kelso Pipe Band play before birthday pupil Ellis Murray cut the ribbon with Councillor Carol Hamilton, Scottish Borders Council’s Executive Member for Children and Young People.
The historic Kelso High flag was then raised for the first time at the new school by Head Boy Charlie Graves and Head Girl Aimee Martin.
Facilities at the new school include 3G sports and 2G hockey pitches as well as grass playing fields, a 100-metre sprint track, long jump pit and netball court.
An eco-garden pond is included alongside space for an orchard, allotment, outdoor classroom and an arts and craft garden.
A new library and gym is also part of the facilities on offer.
The school was delivered for Scottish Borders Council in partnership with Hub South East Scotland.
Paul McGirk, Hub South East’s Chief Executive, said: “We’re proud to have delivered the new Kelso High School, not only on time and on budget, but to an extremely high standard - ensuring high quality and value for money for our client.
“This project has been a fantastic example of partnership working and will provide a modern learning experience for pupils, as well as being a real asset to the community of Kelso.
“We look forward to continuing this successful relationship with Scottish Borders Council as we deliver future projects together.”
Morrison Construction’s Operations Director, David Wilson, added: “It is a great to be at Kelso High School as pupils and staff enjoy their first day in their new school.
“This new education facility will benefit not only current and future students, but the wider community too.
“It’s also extremely encouraging to know that the project has been able to directly benefit the community, supporting six trainees, eight apprentices and creating 45 new jobs during construction.”