Thousands of primary schools across England were this week preparing to defy angry Government warnings and boycott next week's Year 6 Sats tests.
There are just two days to go before the exams kick off with a reading test on Monday and a number of reports indicate up to half of schools could take part in the boycott.
Many headteachers spent this week battling with their consciences over whether to refuse to administer the tests at their schools, with decisions expected to go right down to the wire.
Campaigners are determined to force the abolition of Sats and their replacement with a system of teacher assessment.
Heads who have decided to take part in the action, led by teaching unions the NUT and the NAHT, have been organising "alternative assessment" weeks by making children sit last year's papers, or carrying out teacher- assessment exercises.
But others will be making a stand by suspending normal lessons in favour of "creativity weeks", with a focus on fun and enjoyment.
The final choices heads make could largely depend on the support they have from parents, governors and local authorities, all of whom could apply pressure in either direction.
School leaders will also be contemplating the prospect of losing pay for opting out of tests, which has been recommended in official guidance to governors.
Some may also be considering Schools Secretary Ed Balls' claim that heads have a "professional and moral duty" to administer the tests.
At last week's annual conference of the NAHT, Mr Balls urged heads to think again about the boycott, although the unions insist the action will not be called off unless the Government has a sudden change of heart on the whole issue of testing. There are also concerns the Government could launch a legal challenge to the action, but this is thought increasingly unlikely.
As the pre-boycott tension builds, the NUT is hoping to attract more media attention by hosting an "Anti-Sats picnic" at Jubilee Gardens, near the London Eye, on Sunday.
A flyer for the event promises "food", "storytelling" and "no revision", with participants expected to bring their favourite books.
Mick Brookes, general secretary of the NAHT, said the time had come for heads to "make a stand".
He said: "At our conference sessions last week we had a hugely enthusiastic response, with some branches reporting 100 per cent participation in the action.
"This is a real moment for the profession. We need to stand up and be counted on this issue, secure in the knowledge that we have tried every avenue to effect negotiated change."
But amid the excitement, another major teaching union, the NASUWT, has warned of a number of concerns surrounding the boycott.
General secretary Chris Keates said she had received reports that some headteachers were planning to be "absent" during the tests, leaving their deputies to make the decision about whether to go ahead.
She also expressed concern that extra workload would be forced upon teachers, with some asked to undertake alternative assessments, mark internal tests that have been set to replace Sats, and mark or moderate papers from other schools.
She said there was "a real possibility" of a school facing a formal dispute with her union if its members were put under pressure to do additional work.
She said: "It is unacceptable to take industrial action on the back of an expectation that the workload of staff not taking action will increase.
"It is equally unacceptable to pass the buck to others by planning a `diplomatic absence' during the time the Sats are scheduled.
"Headteachers who decide to take action must do so on the basis that they accept the consequences of the Sats not going ahead."
Original paper headline: Ticking clocks and tinderboxes in countdown to Sats boycott