A fifth of teachers advised to shield by the NHS were asked to return to school last week, according to a new survey.
A snapshot poll by the NASUWT teaching union suggests that 20 per cent of clinically extremely vulnerable teachers were asked to work on site when schools reopened to more pupils.
The survey, which was carried out between 4 June and 7 June, received more than 20,000 responses from NASUWT members.
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In the poll, some 5 per cent of teachers said they had been advised by the NHS that they were clinically extremely vulnerable (shielded) and that they should not leave their home.
Of these, 20 per cent said they been asked to attend their school "nevertheless".
Patrick Roach, NASUWT general secretary, said some schools had been "acting as clinicians", making "life or death decisions" for teachers.
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"I have to say, it's been extremely distressing to hear from teachers, not just in this survey but through our call centre, who are having to enter into, if you will, a negotiation with their employer over whether or not they need to be in the workplace," he said.
"Some schools are saying: 'Well, the government shielding arrangements are pretty much coming to an end, so you'll need to be back.'
"We've had school employers acting as if they were clinicians who could make life and death decisions about whether it's safe for an individual to be in the workplace."
He added: "It's almost as if some employers are insisting that these teachers need to be in schools because they can't be trusted, quite honestly – that's what it seems to boil down to. And that is just simply unacceptable.
"But it has caused huge distress for those teachers, some of whom have said to us – and indeed we've had one or two of them who have presented to us and said: 'I can't take any more of this, I'm resigning.'
"We're in the heart of a recruitment and retention crisis in the profession, and we've got teachers who feel compelled – pushed over the edge – compelled to say, 'I can't take any more of this – this is a matter of life or death for me. I'm shielding, I'm clinically extremely vulnerable' – or clinically vulnerable in the case of many others – and saying, 'I've got to put my health and wellbeing first, and I'm going to quit the profession that I love.' And that, frankly, is not acceptable.
"I've said to [education secretary] Gavin Williamson that he's got to call out those practices, and say that they simply won't do.
"We need to have compassion for those individuals who are going to be more vulnerable if they were to contract this virus. And at the moment compassion seems to be in short supply in some schools."
A Department for Education spokesperson said: "Last week marked the first, cautious step in a phased approach to bringing more children back into the classroom, with secondary schools and colleges beginning to provide some face-to-face support to more students from Monday.
"We will be supporting primary schools that have capacity to bring back more children – with reduced class sizes of 15 – to do so if they can before the summer holidays, and we will be working with the sector to bring all children back to school in September."