Children due to start at a new school in September will be bussed to another site nearly 10 miles away for the first two weeks as the building will not be ready.
The Katherine Warington School near Harpenden was due to open for the start of term on 5 September.
Instead, 180 Year 7 pupils will be taken by bus to the University of Hertfordshire’s law school in Hatfield.
Hertfordshire County Council called the plan “disappointing”.
The secondary academy is being built between Wheathampstead and Batford due to increased demand for school places.
The plan is to build the sports hall first and convert it into nine classrooms while the rest of the school was built.
With building work behind schedule, children will travel about 15km (nine miles) away, with the cost of transport paid for by the Department for Education.
The school said the sports hall will be ready by 19 September with the children moving in on 30 September after a week’s camping trip.
Tony Smith, head teacher, said it “definitely will be ready”.
“We’ve sat down with [builders] Kier… they are absolutely confident that it is totally achievable within the parameters that they’ve set and the building will be finished and handed over [on the new date],” he said.
Mr Smith said parents had been aware of the delay and the school had “mapped out what the contingencies would be”.
“From that point of view they were aware of this before they accepted a [place] at the school,” he said.
A Hertfordshire County Council spokesman said it was “disappointing news for pupils, their parents and the county council”.
It was helping to organise the transport and would work with the school to ensure contingency arrangements “run as smoothly as possible”.
Building firm Kier has been asked by the BBC for comment.