The school’s current Head Boy Charlie Bisset and Head Girl Katie Best jointly carried out the sod-cutting ceremony, ahead of the construction phase of the project beginning on Monday.
They were joined at the Nethershot site of the new school by 12 pupils from the six primaries which make up the Kelso High cluster area – Broomlands, Edenside, Ednam, Morebattle, Sprouston and Yetholm.
The new facility is expected to be completed by autumn 2017, with the official opening to follow.
The building will be able to accommodate 780 pupils, with a wide range of modern facilities for students and the community.
These include 3G sports and 2G hockey pitches as well as grass playing fields, a 100metre sprint track, long jump pit and netball court.
An eco-garden pond is included in the plans alongside space for an orchard, allotment, outdoor classroom and an arts and craft garden.
A new library and gym will also be part of the new school project, which had been delayed by a European ruling affecting ten schools and two health centres across Scotland.
Councillor Sandy Aitchison, Scottish Borders Council’s (SBC) executive member for education, said: “This is a historic and exciting moment for Kelso.
“There is a long tradition of education in the Kelso area, with the current high school building on Bowmont Street serving generations of pupils well.
“However, it is now almost 80 years old, which is why the council decided to take forward this project.
“There has been a long wait to get to this stage, but the sporting and educational facilities that will be offered for pupils and the community at the new high school will create a tremendous environment for all.”
The school is being funded by the Scottish Government through the Scottish Futures Trust. It will be delivered by SBC in partnership with Hub South East Scotland and its appointed contractor Morrison Construction.
In addition, Galliford Try’s Facilities Management business will provide ongoing hard facilities maintenance and lifecycle management to the school over the 25-year concession period.
Paul McGirk, chief executive of Hub South East Scotland Ltd, the Council’s development partner, said: “It’s fantastic to mark the start of construction at the new Kelso High School, allowing us to realise opportunities during the build for people in the Borders – from school and further education visits, to training and work placements.
“Continuing our successful partnership with Scottish Borders Council, we look forward to delivering a first-class, modern facility, designed to meet the needs of pupils, staff and the local community.”
Derek Scotland, project director at Morrison Construction, added: “This is an exciting new development and we’re delighted to be an integral part of it.”