Newspaper headlines can be misleading. It’s the same with headline figures quoted by politicians: you have to read the small print to learn what is really happening.

So, 22.5 hours of class contact time for all teaching staff in Scotland, starting this month, is a nice headline figure. But the details are more complex and less appealing from a school’s viewpoint, says Lindsay Roy, headteacher of Inverkeithing High in Fife and past president of the Headteachers’ Association of Scotland.

“We’re concerned that a gulf does not appear between the rhetoric and the reality,” he says.

“We appreciate the strategic investment the Scottish Executive has made in this. But what’s disappointing is that local authorities, at the same time, are making cutbacks in funding to 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. The only area in which a school can make that kind of cutback is staffing.”

Just before the summer break, the HAS produced a paper on the likely impact of the second phase of reduced class contact time, based on evidence from its members around the country. This concludes that cutbacks which local authorities are imposing means schools in most areas will have insufficient staff to deliver the 22.5 hours limit to every teacher.

The “inevitable conclusion”, says the HAS, is that the shortfall will have to be met by using senior management for teaching, increasing class sizes, reducing absence cover, squeezing the curriculum, or all of the above.

The impact was beginning to be felt in schools towards the end of the last session.

“Timetables planned back in January and February have to reflect available funding and staffing,” says Mr Roy. “So, contrary to what everybody in education wants, class sizes in a number of schools have been increasing, while options available to pupils have been decreasing.”

Teachers and teaching unions confirm that delivery of reduced class contact time is not in itself a problem in schools.

“It is not yet an issue with our membership,” says Jim Docherty, assistant secretary of the Scottish Secondary Teachers’ Association. “As long as it’s understood that teachers will teach to a limit of 22.5 hours a week, including any cover time, it won’t become one.”

Teachers’ unions are taking the line that reduced class contact time is a contractual commitment which must be delivered, while the impact on the whole school, at a time of funding cutbacks, is a management issue.

However, two of the four ways the HAS has identified for schools to meet the predicted staffing shortfall - bigger class sizes and a squeeze on the curriculum - will affect teachers and pupils directly. Indeed, they already seem to be doing so.

“The reduced class contact time isn’t an issue among the teachers I talk to, and I’m certainly getting mine,” says Carolyn McInnes, who teaches French at Eastbank Academy, Glasgow. “We are concerned about class sizes, though.”

Certain parts of the curriculum are more easily squeezed than others. At Dumfries Academy, it is the upper school that feels the pinch, says physics teacher John Lethbridge.

“I’ve got my non-contact time and a bit over, so if colleagues are absent I can supervise their classes,” he says. “But part of the reason for that slack in the system is that we haven’t timetabled an Advanced Higher physics class this year. It’s a bit of a sore point, because if you do run the class, there are always late applicants, especially from other schools.

“What happened before when we hadn’t timetabled Advanced Higher was that parental pressure led to a retired teacher running a class, and that is happening again this year. So, although the non-contact time itself is not an issue - either with me or any of the teachers I talk to - that does conceal other problems in the system.”

The HAS’s third option for delivering reduced class contact time in straitened circumstances - less absence cover within a school - seems to be a ticking time-bomb. While the current message from secondary teachers is that non class contact time is not an issue, there is an equally clear caveat: ask again in a few months, when stricken teachers start to stay at home and the pressure on the system has increased.

At Hermitage Academy, near Helensburgh in Argyll and Bute, about 100 periods a week are available to cover absences this session, says Jim McGonigle, the principal teacher of history. “That’s after everyone has got their non-contact and management time.

“But that only covers three teachers off at once. So, come November, I don’t think there’s any doubt, we will have to find supply teachers from outside.

“But they are very thin on the ground. It’s not so much a supply pool, more of a damp patch!”

Teachers can’t be expected to give up their management or non-contact time, so that leaves senior management staff with very few options, says Mr McGonigle.

“I imagine what will happen is that classes will be sent to a common area and supervised by a senior member of staff.

“There is no doubt there is a problem. HAS says authorities are not providing enough money to implement the national agreement fully. The authorities say they are working within the financial constraints imposed by the executive. And so it goes round.

“Management at my school is sympathetic to the issues that will arise this session. Teachers don’t want a confrontation over non-contact time, which all sides have signed up to, but I’m not sure how the problems are going to be resolved.”

Bill McGregor, the HAS general secretary, welcomes the additional funding for reduced class contact time, announced this summer by the executive, “But there is a continuing problem,” he says. “All around the country local authorities are making savings in school budgets.”

Variously referred to as management, efficiency target, procurement or cash target savings, these local authority cuts are not new, but their effects have been building and are about to become manifest, says Mr McGregor.

“In past years you could usually find the money under various budget heads, like continuing professional development or absence cover,” he says. “But there is no slack left in the system. It’s right down to the bone.

“So, the message we are getting very clearly is that heads are having to look at staffing cuts.”

Every local authority has pledged to provide for reduced class contact time, says Mr McGregor, but because they are also being forced to make savings, very few have put in place additional staffing. Before the summer break only three authorities - Glasgow, Aberdeen and the Western Isles - had done so.

“So, elsewhere there was going to be a shortfall, which would have to be met in schools by existing staffing. Now, they could do that for a time, but not for very long,” says Mr McGregor.

“It wouldn’t even take a flu epidemic. Natural wear and tear would mean schools in these authorities would soon use up their internal capacity.

“Will the additional funding from the Executive solve this problem? Come back in November and we’ll tell you.”