Headteachers could be “called upon” to take national industrial action over “massive cuts to education spending”, the NAHT’s departing general secretary has warned.

Russell Hobby used his final address to the union’s conference to raise concerns about how schools are held to account, and criticise government plans for new grammar schools.

He told delegates he was proud that his seven years in charge had seen the first time the NAHT had taken national industrial action. “We may be called upon to do so again to defend against massive cuts to education spending that will harm standards of education.”

He said it was the “duty of the profession” to tell parents about the full extent and impact of the cuts.

Yesterday, delegates supported a motion saying schools could consider cutting the school week to four and a half days because of funding pressures.

However, Mr Hobby warned against blaming the proposed national funding formula for the funding squeeze, saying: “It is not the method of distribution that we should fight but the amount of money available to begin with.

“No formula can be truly fair if there isn’t enough to go round in the first place.”

He used his farewell speech to dismiss the government’s plans for new grammar schools as “a little gift to the elite disguised in the trappings of populism”, and described it as “a policy that will manifestly and conclusively make things worse for the majority of students”.

Yesterday, delegates called on the union to “campaign vigorously” against the proposals.

Mr Hobby also highlighted the government’s proposals to reform primary assessment, including the end of key stage 1 SATs, and welcomed a reception baseline.

He acknowledged scepticism about the proposal, but told delegates it was worth “serious consideration”.

“For a reception baseline to work, however, it needs at least two features built irrevocably into its design,” he added.

“It must not be used to hold the early years to account. And it must not be used to track and predict individual pupil performance.

“I believe these safeguards are possible and that NAHT should lead on designing them in.”

He raised particular concerns about the way schools are held to account.

He told delegates: “Increasingly I am of the belief that the fundamental problem in our system is not so much assessment itself but the accountability attached to it.

“A well designed assessment can be a vital tool for teachers. It is crude accountability which distorts and corrodes the integrity of assessment.”

Telling delegates that “we have confused the map with territory”, he outlines three rules he said should be laid down as a marker for any accountability system:

• We should never use raw data alone to judge organisational performance;
• We should never make decisions on a single year’s performance;
• We should always offer support before sanction.