Individual teachers are using the new contracts, currently being introduced throughout the further education sector, to negotiate significant - and often secret - overtime payments with principals. Staff running popular day courses are being offered hourly rates of around Pounds 14 to take evening classes.
Some overtime payments are sanctioned by the Sixth-Form Colleges' Employers' Forum, provided evening and weekend work takes a lecturer over the 1,265 hours he or she is contracted to work. But some of the deals infringe the forum's guidelines because they have not been openly acknowledged.
The system of overtime payments has also put the employers on a collision course with the unions which are opposed to any form of overtime money.
Peter Smith, general secretary of the Association of Teachers and Lecturers, has condemned the entire practice. "Teaching should not be a clock-on, clock-off culture and there is no guarantee that overtime payments improve salaries in the longer term."
Too often, arbitrary arrangements to suit short-term needs on selected courses meant some staff were rewarded while others were not. "We have a thorough dislike of secrecy," said Mr Smith.
An investigation of one-in-six colleges by The TES has revealed that two-thirds of those which run evening classes are paying or proposing to pay some staff up to Pounds 42 for three hours' evening work on top of their basic pay.
The findings come in the wake of an announcement by education minister Tim Boswell last month that Pounds 50 million would be withheld from the total budget for FE and tertiary colleges if they fail to get lecturers on to flexible contracts.
He praised the Sixth-Form Colleges' Employers' Forum for its success in getting 82 per cent of colleges on to the new contracts - a move which has sparked a rapid rise in adult learners in the sector. The survey shows that flexibility has come - but at a price.
The contracts stipulate staff must agree to work up to two evenings a week, if required, in return for time off in lieu of day work.
While some principals have hired staff to take evening classes or thrown open the possibility of additional work and salary to existing staff, the survey showed a significant minority had negotiated deals with individual teachers.
Sue Whitham, secretary of the SFCEF, said the new contracts allowed for extra work in several ways, including payments through salaries or contracts outside the standard 1,265-hours agreement.
"Sometimes they have to recruit people separately. That means we are increasingly having to look at new contracts for hourly-paid staff." Further guidance on overtime and hourly rates would be published with an employers' guide to part-time contracts following consultations in the spring, she said.
The principal of one northern sixth-form college said some established teachers taking popular courses had volunteered to run evening classes on top of their daily timetable for additional money.
But, he said: "It is a sensitive issue, at the moment we are quite happy to regard our day and evening courses as distinctive. If they led to the same level of activity I could see it becoming an issue of fairness."
A Midlands principal admitted one experienced teacher of a popular day-time specialist course was being paid extra to work in the evening - unknown to the rest of the staff. "I do not wish to identify the person because of the potential split in my staffroom," she said.
A north-western college principal said one teacher worked in the evening for additional pay, but denied the arrangement was clandestine: "It is obvious what this member of staff is doing," he said.
"However, there are sensitivities involved for colleges, particularly if staff are working different hours and for different rates of pay."