The county council decided this week to choose an eight-company consortium. The deal should be signed early in April. Work at Colfox School in Bridport should begin in July, and the buildings should be occupied in September, 1999.
Headteacher Chris Mason said that until then it would be business as usual for the 900 pupils, who are crammed into the "appallingly designed" building originally made to accommodate 650.
The new school will be on part of the original site, which will become a playing field.
Negotiations for the Pounds 11.5 million project began more than a year ago. The deal means that the consortium will build and maintain the school and grounds, and run the catering and cleaning services, for a minimum of 30 years in return for an annual payment from the LEA.
Colfox was the first purpose-built comprehensive in the south-west. During construction the builder went into liquidation and it was finished hastily.
Gillian Shephard, Education and Employment Secretary, said she was delighted by the news of the impending deal: "Both Colfox and Dorset County Council are blazing the trail for others to follow."
Schools minister Robin Squire told a conference on PFI in London that it had taken longer to get the first deals off the ground in education than in some other sectors. But there were major opportunities to improve education's infrastructure in a way that would not be possible under traditional procedures.
"In some ways we have had a stony PFI field to plough in education. The sector is very diverse, with over 24,000 separate institutions . . . It is a mammoth task to ensure that the PFI message reaches all the key players in the sector," he said.