Some head teachers in north Wales have just half a day a week to run their schools after spending most of their time in class, council bosses say.
The pressures are said to be putting people off applying for head teacher posts, according to a report before Gwynedd Council's cabinet.
Councillors vowed heads should spend no more than one day a week teaching under a plan to shake-up smaller schools.
They also heard some lessons include pupils from four different year groups.
Cabinet members agreed there should never be more than two different year groups involved in a lesson, according to the Local Democracy Reporting Service.
They decided to adopt a new strategy following a two-year consultation with teachers.
Head of education Garem Jackson told the cabinet meeting last week: "We have a number of schools where head teachers are having to teach for four and a half days a week, leaving only half a day to lead the school.
"The size of the school is so small they can often teach four different age ranges, which isn't possible in the long term," he added.
Mr Jackson said recruiting head teachers was a real problem, with deputy heads unwilling to seek promotion and few applicants from outside Gwynedd coming forward.
Secondary schools had a particular problem with some teachers taking lessons in second or third subjects outside their area of expertise, leading to concerns about the quality of children's education, the report said.
One future model for primary and secondary schools could see them join forces in federations led by a non-teaching head teacher and a single governing body, with individual schools and buildings remaining open.
Councillor Gareth Thomas, the cabinet member for education, said teachers were "totally dedicated to their work" and promised any changes would be introduced gradually.