From the start of the new session, teachers' time in front of a class is limited to 22.5 hours a week for all teachers in primary, secondary and special schools, one of the final building blocks to be put in place as part of the national teachers' agreement.
But headteachers say the changes could not have come at a worse time because the squeeze on council budgets means that less money is going into schools.
"I know of schools that have been told they will have the funding for class contact time but they also have to save tens of thousands of pounds,"
Lindsay Roy, headteacher of Inverkeithing High, says.
Mr Roy, a former president of the Headteachers' Association of Scotland, adds: "The only area in which a school can make that kind of cutback is in staffing."
The money made available to implement the teachers' agreement is currently at the centre of a wrangle between local authorities and the Scottish Executive. Ministers have committed pound;32.5 million this year, rising to pound;44 million next year, to ensure they reach their target of having 53,000 teachers by next September.
The executive suspects, however, that some of the cash may have been diverted to other areas and has now imposed conditions to ensure it goes to schools.
An HAS survey of members found that local authority cutbacks mean that schools in most areas have insufficient staff to implement the limit. Heads say the "inevitable conclusion" is that the shortfall will have to be met by using senior management time for teaching, increasing class sizes, reducing absence cover, squeezing the curriculum - or a combination of all of these.
The effect was beginning to be felt at the end of last session. "Timetables planned back in January and February have to reflect available funding and staffing," Mr Roy said. "So, contrary to what everyone in education wants, class sizes in a number of schools have been increasing, while options available to pupils have been decreasing."
Bill McGregor, the association's general secretary, said that, because authorities have been forced to make savings, very few had arranged additional staffing to cover reduced class contact. Only Glasgow, Aberdeen and the Western Isles had done so by the end of the summer term, Mr McGregor claimed.
There is also concern about what happens when the traditional period of high staff absence hits schools during the winter months. Jim McGonigle, principal teacher of history at Hermitage Academy in Musselburgh, suggests the supply pool will become drained.
Mr McGonigle feared the only way schools could cope with shortages would be to send classes to a common area to be supervised by a senior member of staff.
But Greg Dempster, general secretary of the Association of Heads and Deputes in Scotland, which has been a vocal critic of the first phase of reduced class contact introduced in 2004, says there are grounds for believing things might not be so problematic this time because of the continuing increase in teacher numbers.
"Reduced class contact is a great thing for teachers and for education generally," Mr Dempster said. "So we are hopeful this phase might work out better than the last one."
The Educational Institute of Scotland, which regards itself as something of a guardian of the national agreement, also take an upbeat view, funding problems notwithstanding.
Drew Morrice, the union's assistant secretary, said an evaluation of what worked last time (such as the use of visiting specialists to fill the time left) and what did not work (such as whole-school assemblies) should actually make it easier this time.
Scotland Plus 2-3