The project was originally due to cost £21m to build but the budget has been slashed by 30% as part of Government spending cuts.
Construction will start in July and is due for completion in September 2013.
This new site will feature a two-storey 950sqm atrium, an outdoor terrace and new science and technology departments.
A new sports hall, fitness suite and drama studio will also be created.
Kier will work with Mace and Gloucester architects Roberts Limbrick on the development.