Teachers will assess the academic standards of seven-year-olds under a radically different marking system starting this year, officials revealed this week.

Many primary schools in Wales started teaching the “revolutionary” foundation phase for under-5s for the first time last week as children arrived in reception classes. The new play-based curriculum will be introduced nationally to all under-7s by 2012.

But it has emerged that teaching staff will have to get to grips with a new marking technique: pupils’ performance will be tracked more closely from the age of three and in regular consultation with parents.

However, critics of the new system of assessment say it could mean that the success of the foundation phase in raising literacy and numeracy standards, compared to the former key stage 1 curriculum, will be impossible to measure.

David Reynolds, professor of education at Plymouth University, said it would be difficult to judge. One option, he told TES Cymru, would be to compare children in Wales with those in countries on which the phase has been based, such as Denmark or New Zealand.

“I think that such a lot of political and emotional investment has been made in the foundation phase that it would be inconceivable that they wouldn’t look in detail at the results and see whether there are improvements,” he said. The KS1 assessment results of seven-year-olds from 22 pilot schools were omitted from the 2008 results published by the Assembly government last month.

A government spokeswoman said pupils in pilot schools had not been taught the KS1 national curriculum so it was impossible to compare like on like.

“They simply cannot be assessed against the KS1 curriculum,” she said, adding that it was now crucial for nursery staff and teachers to start monitoring pupils as soon as they start in education to provide a “baseline assessment”.

When pupils reach seven, the last statutory year of the phase, their original score will be compared with their performance in four categories: personal and social development; wellbeing and cultural diversity; mathematical development and language; and literacy and communication skills in English or Welsh.

“Eventually this will replace the national reporting of the performance of seven-year-olds,” said the spokeswoman.

The foundation phase has been hailed as the best way to raise flagging literacy and numeracy standards. But there are concerns that poorly trained teachers will not deliver the balance between structured and unstructured play, which is needed for effective learning.

Experts say the best adult to pupil ratio for the foundation phase is one to eight for under-fives and one to 15 for under-7s.