New national tests for primary and secondary pupils are to be changed after only one in 10 passed one exam, The TES can reveal.

The Government is to respond to the unexpectedly low scores by altering the tests so that pupils get more time to complete the higher-level sections. Procedures will also be tightened to make sure that the right students sit the tests.

More than 400 schools have been taking part in a two-year pilot of the new tests, which are expected to replace Sats from 2010.

The TES learnt that when they were first trialled in December, fewer than 10 per cent passed the level 6 reading test. Figures were slightly higher for writing and maths.

The Government postponed publication of the results in January because they were disappointing. A full analysis will not be published until autumn at the earliest.

The new tests for key stages 2 and 3 are set at a single national curriculum level in reading, writing and maths; teachers enter pupils when they believe they are ready.

In December, the tests were set at levels 3 to 6. Pass rates appear to have been higher for the lower-level tests, and primary pupils consistently outperformed their secondary counterparts.

Emails between the National Assessment Agency (NAA) and the Department for Children, Schools and Families obtained by The TES confirm that agency officials believed some schools entered pupils incorrectly. One email, dated January 25, said: “The early feedback ... is that schools are quite open about insecure teacher assessment and inaccurate entries.” In some cases, pupils were entered in spite of their teachers’ assessments.

Another email from a department official bemoans the structure of the exam. Instead of facing problems of steadily increasing difficulty, the first few questions may have thrown many pupils, who then failed to finish the paper.

The official writes: “It looks as if pupils faced some of the hardest questions first, with nearly a third not getting as far as the final, easier questions. Is that right?”

Colin Watson, the agency’s head of standards, agreed that this “could have put them off”, but said he was “not sure it would have made much of a difference”.

Before the next round of tests next month, the NAA has produced guidance for local authorities to help ensure pupils only take the tests when their teachers feel they are ready. The time allowed for the level 6 tests will also be increased from 50 minutes to an hour.

Concerns about league tables and exams, including the new when-ready tests, dominated the National Association of Head Teachers’ annual conference in Liverpool.

Heads fight back, page 14.