Some school subjects could be scrapped as a result of the Scottish government's budget, it was claimed yesterday.
The general secretary of the leadership organisation for Secondary Schools in Scotland -School Leaders Scotland - also said the number of pupils per class would also rise, with teacher vacancies to remain unfilled.
It comes as the Convention of Scottish Local Authorities (Cosla) warned of job losses and service cuts in local authorities.
The government says it will work with local authorities to "protect what matters in Scottish society".
Finance Secretary John Swinney laid out his Budget on Thursday, which included a £550million year-on-year increase for local government.
However, analysis published by Cosla suggests the cash increase could be just £71million once Scottish government-mandated policies are paid for, while it claims capital funding will remain flat for next year.
£1billion requested
The organisation had called for a £1billion cash injection to maintain current services and fund pay deals for staff.
Jim Thewliss, general secretary of School Leaders Scotland, said the spending plans would lead to cuts in schools.
"We are now at a stage where we really have a service where if you're going to start to cut any more you're cutting into the basic functions of the service, which is essentially...the number of teachers in front of young people in schools," he told the BBC.
"Vacancies which are already identified within schools - head teachers have been told these are not going to be filled."
Mr Thewliss said teachers across the country were expecting increases in class sizes and said some subjects would be removed if "deemed as non-viable".
He gave the example of higher modern languages subjects, which he said often had small class sizes of four or five pupils.
Even greater pressure
"We're now starting to find even greater pressure coming upon what is deemed as non-viable in terms of class sizes," added Mr Thewliss.
He went on: "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 are now in the position...of managing expectations to a degree. And I've 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."
Katie Hagman, an SNP councillor and Cosla resources spokesperson, told the BBC the Budget announcement had been met with "extreme disappointment" after the organisation said it received less than a 10th of what it had asked for from the government.
She said: "If we got the £1billion that would save, potentially, job cuts.
"What it translates to is equivalent of, say, 17,500 teachers. We're looking at essential services, things like youth work, things like economic development, working with our third sectors as well for domestic abuse and family support services.
Protecting services
"All these are considered essential services and we're at the front line of local government and it's these types of services that we will be desperately trying to protect."
Mr Swinney defended his budget announcement, but acknowledged he had set out a "bleak picture" in parliament.
He told the BBC that the war in Ukraine had increased inflation and the financial stress had been "compounded by some of the significant strategic mistakes" by the UK Government.
Mr Swinney disputed claims the councils had been short-changed, saying their funding had increased by £570million year-on-year.
In response to warnings of service and jobs cuts, he said: "Local government came to me and asked for £1billion and I said up front there was just no way I was going to be able to achieve that. But I have managed to put £570million into the local government settlement.
"I've said to local authorities we need to work together to ensure that we protect what matters in Scottish society, which is preventative early intervention services where we do our best to change the outcomes for people within our society."