Construction will begin next week on St Joseph's College as part of the Dufries Learning Town Project.
Preparation work has already been carried out with the provision of temporary classroom accommodation ahead of the new school term.
The work is part of a £60M programme that will also see a new North West Campus in Lochside to replace Maxwelltown High School.
A new educational Centre will also be built near Cuckoo Bridge.
Graham Construction has been appointed lead contractor on these three phase-1 projects.
Work to St Jospeh's Collge will involve the repair, restoration and refurbishment of the original Grade B listed stone building, which was built in the early 19th Century.
The work will include refurbishment of an existing extension and the demolition and construction of a new wing in order to provide all facilities under one roof.
External works will include the provision of a new "3G sports facility" and the reinstatement of grass pitches.
When completed, St Joseph's will be more than 8,000 square metres in size with accommodation for 644 students and 72 staff.