About 10,000 five and six-year-olds took the phonics screening check this week, amid mounting criticisms of the policy.
Pupils taking the test must read aloud a list of 20 words and 20 "non-words" to a teacher. The "non-words" ensure that children are decoding and not repeating words they have learnt already.
Steve Iredale, vice-president of heads' union the NAHT, observed a test being carried out in Loughborough.
"For internal purposes what I have seen could be very useful, but many schools will already have excellent tracking systems in place," he said. "What creates difficulties is that whatever is done by pupils goes to the Department for Education and becomes another stick to beat people with.
"Why not just let schools use it as an option?"
The test is due to be rolled out nationally next year - the cost of the policy has not been released although the trial is costing #163;250,000.
It is expected to take about five to ten minutes per pupil and the results for each pupil will be given to parents. The school's results will be recorded on RaiseOnline and available to Ofsted for use in inspections but will not be published in performance tables. National and local authority results will be reported.
Guidance on the tests says that the policy is aimed at encouraging schools to pursue a rigorous phonics programme but that this does not mean schools should delay teaching pupils wider literacy and comprehension skills.
One head who took part in the pre-trial tests said at the time that her teachers found it a "simple but helpful assessment".
Meanwhile, more than 1,200 people have signed a petition against the check launched by early-years organisation TACTYC.
Emeritus professor Janet Moyles, who begun the petition, said: "Early years teachers are very keen to do what they are asked to do.
"If they feel under pressure and Ofsted gets involved in it, then the conscientiousness of early years practitioners mean they will try and make sure children will do well in the tests and I think that will skew the pedagogy and the curriculum. I don't see how it can't."