Many heads will feel "frustrated and impatient" over a decision to call off plans for a boycott of Sats tests, the leader of heads' union the NAHT has admitted.
But Russell Hobby, the new general secretary, insisted his organisation's decision to ditch plans for a ballot after the Government offered them an independent review of Sats last week is "the best way to change things".
He said Education Secretary Michael Gove had only come forward with the offer because last year's boycott had been so successful.
Christine Blower, general secretary of the NUT, which also took part in the boycott, "welcomed" the review of assessment and would engage with it but insisted it must be "open and independent". However, she declined to rule out a future boycott, saying nothing was "off the table".
The news comes as many heads had been limbering up for major confrontation over Sats for the second year running. The decision could lead to schools going it alone, running local ballots and boycotts.
Mr Hobby said a second boycott next May would only have served to "harden the attitudes" of ministers, who are still strongly in favour of both testing and league tables.
Mr Gove's offer of a review, and the NAHT's swift acceptance on a vote of 34 votes to two, will come as a surprise to many as a ballot of members over a boycott had looked almost inevitable until last week.
In a letter to Mr Hobby, Michael Gove said he would like to "see if we can reform our system of assessment and accountability to take account of those concerns that have been raised by committed professionals".
The review, which will report next spring, will "look broadly at the testing and accountability system for primary schools," he said. It will consider how to ensure accountability to pupils and parents while avoiding teaching to the test and the use of results to judge schools.
Mr Gove asked that, in exchange for the NAHT's full involvement in the review, the union should support the administration of the 2011 tests.
Mr Hobby, a former management consultant, denied that he was backing down by choosing to negotiate. He said participating in a review would also win the union more influence in Government on a whole range of other issues, from Ofsted to school funding.
"We know the Government doesn't agree with us; they think tests are great and league tables are better," said Mr Hobby. "If we go to a boycott it will confirm their point of view, it will harden attitudes.
"It's a very difficult thing for me personally. The easiest thing for me to do would have been to do a boycott - it would endear me to the membership, but I think this is the best way to change Sats."
In a letter to members, Mr Hobby and NAHT president Mike Welsh say: "This review is a genuine opportunity for the profession to shape the future of assessment for all children.
"Participating in Sats is unpalatable. The choice before us is not whether the current regime is good or bad. If Sats were wrong last year, they are wrong this year. The choice before us is which strategy will best achieve a change to the present system ... We do think they (ministers) are open to persuasion, through the intellectual and moral case that we will make."
Despite the positivity from the NAHT leadership, many heads will, however, see the review as a Government stalling measure.
Previous reviews have resulted in gains, including an end to blanket testing of 11-year-olds in science this year, but none has gone as far as prioritising teacher assessment or axing league tables.
Mike Kent, headteacher of Comber Grove Primary School in Camberwell, south London, who took part in last year's boycott, said: "It's understandable why the NAHT is now taking this position, but there will be a lot of anger if the review comes out and it all comes to nothing."