The number of primary schools which fall below government floor standards this year will be in the hundreds rather than the thousands despite harder tests at key stage 2, heads have been assured.

Seeking to allay fears that new tougher tests could see tens of thousands of schools deemed to be failing as a result of falling below the floor standards, Nicky Morgan has announced her department will focus on progress rather than attainment and manipulate figures to restrict the number of schools which fail.

Despite the change, hundreds of schools will still fall below the standards this year, but the impact will be far less significant than first feared: 676 primaries fell below it last year, meaning a maximum of 682 will be below it this time.

Primary schools are measured on pupils’ abilities in reading, writing and maths, and previously were considered to be failing if fewer than 65 per cent of pupils failed to achieve a level four in all three disciplines at key stage 2.

The floor standard for this year is yet to be set, but heads had expressed fears that harder tests would mean failure, followed by potential intervention and dismissal of senior staff. Morgan sought to put an end to those fears when at the National Association of Headteachers conference this morning.

She said: “As you know, if a school meets the progress standard it is above the floor altogether. We have made sure all who hold schools accountable are aware of this too, and we will continue to do so.

“Historically, the floor standard has identified only a small proportion of schools every year which are below that standard – and this year I can reassure you that no more than one per cent more schools will be below the floor standard than last year.”

Morgan denied that the higher expectations embodied in the new national curriculum would somehow ‘inappropriate’, and claimed more rigorous tests at key stage 2 would help address the attainment gap between England and other countries like Korea, Singapore and Ireland.

The change was welcomed by Russell Hobby, the union’s general secretary, who said a lot of schools had assumed they were going to fail.

“We’ve learnt very clearly today that there will be – within a one per cent margin of error – no more schools below the floor this year than there were last year,” he said.

“That’s very significant because I think many people assume that there will be tens of thousands of schools failing at the moment and what we know is that it won’t be that. It’s in the hundreds, not the thousands.”

Criticism of the new tests has focused on the spelling and grammar test for 11-year-olds, which critics have claimed are too hard even for some well-educated adults to complete.