After the Educational Institute of Scotland called on members to walk out on Friday afternoons if they have not had their 1.5 hours of free time during the week, the Association of Head Teachers in Scotland said: "This lines up with the way we are thinking."
"It's a nightmare," said Kay Hall, president of the AHTS, whose members are expected to cover if no teachers can be found.
"Number one, we are not teachers any more. Management of schools is extremely hard work. We are going to put limits on the amount of time to cover."
Their proposal is that heads should cover three in five days. After three days they would send a letter to parents saying they would have to send children home.
"We are taking legal advice on this," Mrs Hall said. "The word 'reasonable'
is used in our contract but it is not defined. We think it is unreasonable that in running schools we are still expected to cover for teachers."
The association has written a formal letter this week to Ronnie O'Connor, director of education in Glasgow, where the problem of cover is most acute.
It plans to send it to all other authorities.
"Glasgow is the key area," Ms Hall said, "and we will be getting Ronnie O'Connor to set working time arrangements. This is no longer acceptable."
The row blew up this week when the Educational Institute of Scotland issued advice stating: "If (the non-class contact time) has not been delivered by pm on a Friday then the teacher is entitled to indicate they will be taking the time at 1.30pm that day and alternative arrangements will be required for supervision of their class."
Gordon Smith, vice-president of the AHTS, said: "We would like to emphasise the fact that heads were not a party to the practicalities or arrangements for the 21st century agreement, about what was going to happen when reduced teacher contact time was introduced.
"There is a shortage of supply teachers. One of our members had to cover for 28 consecutive work days last year. We knew this would be exacerbated by the reduction in class contact time but we sit outside the Scottish Negotiating Committee for Teachers.
"At no stage have people sat down with us and said, how are you going to cover this? The chance of a crisis happening in schools is very real, but no one said, is this going to work? This is our main beef."