Pioneering schools have cut teachers' workload but will probably not be able to continue to do so without extra cash, an evaluation of the scheme has revealed.
Pathfinder primary and special schools, piloting the workload agreement, cut their teachers' week by almost four hours to 50 hours on average.
Secondary teachers worked an average one hour less than before the pilot.
The 32 schools bid for a share of pound;4 million to take on extra staff, invest in computer technology or provide extra space or new buildings. For example, Bovingdon primary in Hertfordshire received pound;170,000, Corsham primary, Wiltshire, pound;170,000 and Etone community school, Warwickshire, pound;283,699.
Professor Hywel Thomas, of Birmingham university, who led the evaluation, said in his report that classroom teachers across all types of schools reported a reduction in their hours, but he concluded: "There are concerns about sustainability and whether the additional resources that supported many of the changes will continue and, if not, whether the schools will be able to sustain the changes they have introduced."
The report has consequences for all schools under the workload agreement signed by ministers, unions and employers. David Hart, general secretary of the National Association of Head Teachers, said: "The report shows that there not only has to be additional funding but that it has to continue or the good work of pathfinder schools will be lost.
"The vast majority of schools across the country will be looking for the sort of cash the pathfinder schools were given, particularly in 20056 when the most expensive part of the workload agreement is to come on stream."
The agreement offers teachers preparation and marking time in return for a "remodelling of the profession" with teaching assistants taking more classes.
Under the deal, more than 20 routine tasks have become the responsibility of support staff. A TES survey in January found more than half of teachers still routinely did seven of the tasks, some because of funding problems.
Some changes, for example restructuring the school day, did not depend on extra cash, said Professor Thomas. His report also noted that the hours worked did not necessarily relate to job satisfaction or motivation.
Martin Mangan, head of 450-pupil Bovingdon, created four new posts: two cover supervisors, a bursar and resources officer.
His teachers have one half-day of non-contact time each week. Two cover supervisors take extra-curricular classes for pupils aged three to eight, and part-time language, music and dance teachers take pupils aged nine to 11.
He said: "Pathfinder brought the funding for additional roles. Whether those roles continue when we go back to our normal school budget is a close-run thing."
Some pathfinder primaries cut an average of seven hours from classroom teachers' 50-hour working week. But in three of the 12 secondaries and two of the 15 primaries the teachers' working hours rose.
The report said primary schools may find "quick wins" easier, teachers saved 52 minutes a week by no longer putting up displays. It also found schools where teachers felt valued by their managers had larger reductions in hours.
Christine Folker, head at 150-pupil Sutton Veny primary, Wiltshire, has funded a half-day of non-contact time for her teachers without pathfinder money. She adds to the budget the pound;10,000 she earns annually from consultancy work for the National College for School Leadership and the education authority. This helps pay for six outside experts who take pupils on Wednesday afternoons.
A separate survey of 600 schools by Continental Research found one in 25 primaries has support staff regularly taking whole classes.