Jim Thewliss, general secretary of School Leaders Scotland, said the squeezing of budgets is "now starting to bite" in schools and classrooms.
In a grim assessment, he said staff wanted to promote aspiration in pupils but are now in the position of "managing expectations".
Mr Thewliss made the comments following the publication of the draft Scottish Budget on Thursday. Deputy First Minister John Swinney, the acting Finance Secretary, admitted he had set out “a pretty bleak picture” to MSPs as he published the tax and spending plans.
Cosla, the council body, had asked for £1 billion to address multiple challenges, but said the cash allocated for 2023/24 amounts to an uplift of just £71 million once policy commitments are taken into account.
Speaking on the BBC's Sunday Show, Mr Thewliss said: "There have been year-on-year cuts within the education budget over the past, certainly 10 years, perhaps even longer than that.
"We're now at a stage where we really have a service where if you start to cut any more, you're cutting into the basic function of the service, which essentially...is the number of teachers in front of young people within schools."
He added: "What we've experienced at the moment is that vacancies which are already identified within schools – head teachers have been told these are not going to be filled, we'll start to manage the service to reduce the number of teachers within local authorities.
"In terms of impact on young people, the cuts running up until now to an extent didn't have a direct impact on young people, but it is now starting to bite in the school and in the classroom."
Asked how pupils will experience this, he said: "Things like class sizes starting to increase, certain subjects within the curriculum starting to be removed because they are deemed as non-viable. That's the experience that we're hearing from members across the country."
Mr Thewliss said schools have already been working together to share classes in subjects such as Higher modern languages. He said there is now "even greater pressure" on these subjects.
He added: "We're very much in the business of promoting aspiration within young people. We have bought entirely into the whole notion of young people having the best educational experience and the best life chances that they can have.
"We're now in the position...of managing expectations to a degree, and I have no idea where we're going to move with this, but it's becoming much, much more challenging in managing those expectations at the same time as promoting aspiration."
Mr Thewliss called on ministers and council leaders to work with schools, and insisted school leaders will continue to do the "very best" they can.
On Thursday, Mr Swinney announced people earning more than £43,662 will pay more in tax next year as laid out the Scottish Government’s plans, which had a focus on the NHS and social security.
Speaking to the BBC, he said: “I set out, frankly, a pretty bleak picture to Parliament on Thursday. It was an explanation of the very real difficulties we find ourselves in as a consequence of some of the global issues which we often feel are a long way away from us.
“But on this occasion the war in Ukraine has brought energy and price inflation right to the heart of our economy and public services – compounded by some of the significant strategic mistakes that have been made in the United Kingdom around Brexit and the mini-budget in early September.”
He added: “I think anyone observing my budget statement on Thursday would recognise that I gave a pretty candid, open explanation of the scale of difficulty that we face.”
Mr Swinney insisted Scotland spends more per capita on education than south of the border.
He said the local government settlement had increased by £570 million, and added that he had told Cosla “there was just no way” he would be able to allocate £1 billion.
Speaking to the same programme, Cosla’s resources spokeswoman Katie Hagmann said there was “disappointment across the board” at the settlement for local government, adding that councils would be “desperately trying to protect” frontline services. There are fears of substantial job losses.
Mr Swinney said that, as a result of financial pressure, the country should be open to reforming public services in a bid to save money.
He said: “The financial pressures on all of us because of inflation are so great we have to change the way we deliver public services.”
Scottish Labour finance spokesman Daniel Johnson said Mr Swinney was “out of touch with Scotland”.
He said: “Even SNP councillors are calling out their own party for the failure to support councils after 15 years of savage cuts.
“While claiming to reject austerity, John Swinney is foisting it upon local government – leaving council services decimated across the country.”
Scottish Liberal Democrat leader Alex Cole-Hamilton said the Government “is squeezing councils and education to the pips”.
He said: “Their budgets have been eroded year after year by the SNP and Greens. The consequences of that can be seen in the failure to close the poverty-related attainment gap to the potholes in our roads.
“Local authorities need both a fair deal from the Scottish Government and a power surge that recognises the important work that they do. They need new hope, not another ministerial takeover of social care and a billion pound bureaucracy that would trample over local services once again.”