The school secured £1.4-million from Devon County Council's capital strategy fund last year as part of the capital build programme.
It will be used to replace four outdated classroom buildings with one new purpose-built classroom block. It will have three classrooms, each with separate cloakrooms and toilets, and additional learning space and a new school hall.
School headteacher Heather Poustie said: 'Fencing has been erected around the building site and pupils and staff are very excited now that the dream has become a reality.'
On the first day that building work started, planned football and netball matches against Okehampton Primary had to be re-arranged.
The netball matches were played on the South Zeal Recreation Ground courts, where both South Tawton A and B teams were delighted to win their matches.
A large digger has now moved onto a corner of the school field.
Phase one of the build includes three new classes and a resource base as well as toilets, cloakrooms and stock cupboards and should be ready for the first day of September.
Phase two, which includes the new school hall, should be completed by January 2016.