The builder was signed up more than four years ago to lead the design for Denbighshire County Council.
But the project became entangled in the Welsh 21st century school building spending review.
Willmott Dixon can now go ahead and build the £25m new Rhyl High School on existing playing fields and is expected to start work in June.
The three storeys high will accommodate 1,245 pupils once finished at the end of 2015. Then the current school buildings will be demolished.
The Welsh Government, who along with Denbighshire Council will provide funding for the project, and has confirmed that the business case for the replacement facility had now been approved.
The Minister for Education and Skills, Huw Lewis said: “I am pleased to confirm that I have now approved the final business case for a replacement school in relation to Rhyl High.
“Local Government officers will now be notified by my officials that they have they have the approval to proceed and that a Grant Award letter for Welsh Government’s financial support will be issued.”