Head teachers are warning parents it is "make or break time" at schools in Conwy due to planned budget cuts.
Letters sent by heads from the county's seven high schools claim some face £200,000 in cuts with larger class sizes and fewer staff a possibility.
They said the "situation has become critical" with cuts of 3.6% and 4% being tabled by Conwy council.
The council's education spokesman said it shared teachers concerns but it had to make £15m savings from April.
The letter said: "There simply are not the savings to be made that can make up for the huge shortfall in our funding.
"It feels very unfair to our children and to our staff. The situation is not sustainable."
The letter has been signed by heads at Ysgol Aberconwy, Ysgol y Creuddyn, Ysgol Eirias, Ysgol John Bright, Ysgol Bryn Elian, Ysgol Dyffryn Conwy and Ysgol Emrys ap Iwan.
They have highlighted several areas of concern:
- School buildings falling into disrepair
- Reduction in support for pupils with additional learning needs
- Cuts to pastoral services including teaching assistants and support staff
- Outdated computer equipment
- Fewer trips and after school activities
- Significant challenge to develop Welsh medium education
The council has yet to formally set its budget for 2019-20 with residents facing a 9.6% hike in council tax.
The authority is also looking to cut social care funding by 4% with 7% cuts to all other services, except education which faces a cut of between 3.6% and 4%.
Councillor Garffild Lloyd Lewis, the council's cabinet member for education, said next's year cash cut settlement agreed by the Welsh Government was a "real no winner" for the authority.
"We have had to make some very, very tough decisions," he said, adding that he had been speaking with teachers, governors and unions and "shared their concerns".
A Welsh Government spokesperson said: "Education services for pupils up to age 16 are provided by local authorities from their core budgets, according to local needs and priorities.
"The Welsh Government continues to provide funding this year so that no local authority faces a reduction in their budget of more than 0.3%. Conwy is one of five local authorities which will receive a share of £3.5m of top-up funding next year.
"This is against the background of the UK government's sustained austerity agenda, which has led to a cut of nearly £1bn to Wales' overall budget. We will continue to call for additional resources to be spent on our public services, including our schools."