Willie Hart, EIS local secretary, said he dealt with around one new case a week of parents behaving in an “unreasonable and insidious manner” and the problem was increasing, particularly in primary schools.

The union says a number of parents are by-passing the school and taking their complaints directly to the council. An education department spokesman admitted there had been a “modest increase” in parents’ complaints this session.

The EIS is concerned that even a minor issue which would normally be handled at school generates a major response, involving headteachers, officials and staff in a prolonged and occasionally interminable dispute.

One teacher was off school for eight weeks with stress-related illness after a case dragged on for five months. In another, a parent kept up the pressure on the school even after the child had been taken off the roll.

In a further fracas, a primary head went on sick leave after a parent harassed her by phoning regularly, turning up at school without warning and “hanging around” the playground at the end of the day.

Mr Hart said the treatment of teachers was tantamount to stalking in some cases. “The authority must take action to stop this insidious treatment of teachers by vexatious parents. This misplaced form of parental involvement is part of the culture of criticising public employees and our members are making it clear to us that they expect more protection from their employer,” he said.

The union is to raise its concerns with the authority at the next joint consultative meeting in February and will submit a motion to the institute’s annual conference in June.

“We must raise this as a public issue,” Mr Hart continued, “so that the minority of parents who behave in an unreasonable manner are called to account by the majority who want a genuine partnership. The EIS has taken civil action against parents in the past, and may have to do so again, but this should be the job of the authority.”

Maire Whitehead, vice-president of the Association of Head Teachers in Scotland and head of St Mirin’s Primary in the city, echoed the concerns. “Parents frequently make attempts at intimidation in one way or another. Teachers do get threatened. You often hear about intimidation of medical staff but people should not think it’s confined to the health service. More and more people are not willing to accept any kind of authority and intimidation is more overt nowadays,” Mrs Whitehead stated.

Teachers, she believed, sometimes feared walking home because of the physical threats, cars had been vandalised and others had been phoned at home. “Perhaps it’s time teachers stood up and admitted it happened,” Mrs Whitehead added.

Government ministers who repeatedly blamed schools for failing and criticising teachers had to take their share of responsibility.

A council spokesman confirmed the trend but said there had been no serious cases. “This is not just a Glasgow problem. While it is the case that parents are more voluble than before and are encouraged to take more interest in their child’s education, the downside is the number of unreasonable complaints we are receiving,” he said.

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Judith Gillespie, Scottish Parent Teacher Council development manager, said there was anecdotal evidence that some people had “completely unrealistic expectations” of what their children could achieve at school and what was happening in Glasgow was probably only the tip of the iceberg. “The problem is caused by the ethos created by the previous government which cast parents in the role of customers and consumers, rather than partners. So far the present government has done nothing to unpick this.”