The clauses in a handful of private finance initiatives raise questions about how the Government can extend school opening hours when it also plans to rebuild half of secondary schools through PFI.
The 6pm lockings have been introduced at seven primary and special schools in Lincolnshire which have been built by Bovis Lend Lease, a London-based international building company behind 14 school PFI schemes in England.
Under PFI, private companies construct the buildings and are then paid to manage their cleaning, catering and other services, usually for 25 to 30 years.
Christine Wright, head of St Botolph's Church of England school in Sleaford, said she was very pleased with many aspects of the company's work. But she had found the limitations on opening hours increasingly frustrating.
The school has to apply at least a fortnight in advance if it wants to book an "additional school period" to extend its closing time by two hours, and needs to apply for two if it wishes to remain open past 8pm.
Staff are not allowed into the building before it opens at 8am or at all during the holidays, unless the school books additional time.
Mrs Wright said: "They've done a good job with the buildings. What is frustrating is that the contracts are so rigidly enforced, which means we can't stay on site if we have work to do or prepare our classrooms in the holiday unless we've booked ahead."
Bovis Lend Lease said that the agreed opening times for its schools depended on agreements with local authorities. While its Lincolnshire schools closed earlier, its six schools in Newcastle stayed open until 10pm for adult education classes.
A spokesman for the company said that, in the case of St Botolph's, the council had agreed that 8am to 6pm was "sufficient because there's little going on outside those hours".
Because the school wanted to open some of its facilities to the community, St Botolph's had been granted 140 "additional school periods" while others in the authority had been given 40.
"As there are only 190 school days it means the head could keep her school open to 8pm almost every day, if she applied for it," the spokesman said.