Teachers have missed out on hard-earned sabbaticals because their heads have grabbed the breaks for themselves.
A study into a government project that funds sabbaticals for experienced teachers found that headteachers would sometimes take the breaks without informing their staff that they could apply.
An anonymous head quoted in the report said: "In a way I kept it to myself.
I saw it and thought 'I'm gonna jump on that!'"
Researchers from the National Foundation for Educational Research were generally positive about the Department for Education and Skills sabbatical project, which ends this month.
The scheme, launched in 2001, was intended to provide 4,000 experienced teachers in challenging schools with six-week breaks to work on projects which would help their professional development.
However, the researchers found that teachers who received the breaks were often reluctant to leave their schools, in some cases because they felt guilty about letting down their colleagues.
Around a quarter of the teachers took their sabbatical in their own school and nearly 30 per cent never took more than one day at a time off work.
One teacher, who found it difficult to work away from their classroom, said: "I didn't realise just how institutionalised I had become. I felt I had to keep showing my face in school - I thoroughly enjoyed the sabbatical but it took me a long time not to feel guilty. I felt I was playing truant."
The DfES provided pound;6,000 for each sabbatical to pay for supply cover and any costs incurred to the teacher and the school.
But the researchers said that in five of the 130 cases they examined there was "no evidence that a sabbatical, if defined as time taken out of normal duties, had taken place" and that some schools were simply pocketing the money.
The report said: "One head refused to permit participating teachers to be interviewed, and suggested they would not have been aware that they were taking part in a sabbatical scheme."
The researchers said the sabbaticals had produced "highly desirable outcomes" for both the teachers and their schools, despite the extra work reported by some.
Of those who took sabbaticals, 87 per cent said they felt refreshed, more confident and knowledgeable when they returned.
A National Union of Teachers spokeswoman said it was a shame that teachers felt so reluctant to leave their classrooms. "It's difficult to persuade them that they ought to, but when they do take a sabbatical they realise the benefits," she said.
Sabbaticals for teachers: an evaluation is at www.dfes.gov.uk