The head of the NASUWT teachers' union, Chris Keates, has accused the Education Secretary Michael Gove of “recklessly pursued a relentless attack on the profession".
He called on Mr Gove to respond to the union's "reasonable demands" in order to avoid "widespread disruption in schools" this summer.
NUT head Christine Blower warned that the pensions situation and increased workload is "making teaching an unsustainable option for many".
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The event, which was held at the Dorchester Hotel last Thursday, was attended by representatives of some of Britain’s biggest companies.
Reports in the Daily Mail and the Telegraph said published guidance has been changed for 2018 to only fly the Union flag on Remembrance Day.
The author, who explored feminist themes, was best known for her Earthsea books.