Pupils could be taught online for one day a week as a way for schools to manage their rising heating bills, a council has suggested.
Wearing coats in class and leaving staff positions unfilled are also among the possible cost saving measures sent in a briefing to headteachers in Powys.
Every option must be considered to deal with a financial crisis, said the county's cabinet member for education.
But each school will be left to decide for itself how to make the savings.
Pete Roberts, the Powys cabinet member for education, told a meeting of the council that school budgets were being looked at "in detail" for "potential solutions".
"We did suggest the possibility of a four-day week," Mr Roberts said, "with a fifth day being taught virtually as well as blended weeks of learning as extreme cases for consideration."
