Glasgow is the key area for action, because of the acute shortage of supply teachers.
Sandra West, headteacher at Sunnyside primary, said: "I had to wait two weeks, three days to get cover for our long-term cover - it was constant phoning."
Bill Roddy at Nitshill primary is "dead against timetabled assemblies for the sake of providing cover". With 87 pupils across four classes, he has a teacher who is not class committed and can cover.
"It's a game of give and take, co-operation and goodwill," he says. "If a person is timetabled to have time off on a Monday I try to make it up later in the week."