All the number work is taking place in Welsh, even though most of the children are from English-speaking backgrounds.
"This is where the miracle takes place," says headteacher Derfel Griffiths of the reception class where the four-year-olds are playing and learning with over-sized dominoes.
"All the infants are taught in Welsh with only a little English. Many come here with no Welsh and learn the language, as well as how to read, write and count."
It was quite different when Mr Griffiths was at school. Despite being a first-language Welsh speaker, growing up in Anglesey in the 1970s, he was taught maths in English in primary school.
"All oral work was done in Welsh, and all written work in maths and science was in English. Luckily the swing to teaching in Welsh had happened before I started teaching in 1992."
On the floor, reception children are learning to count with coloured wooden blocks of 10 to which they add single numbers in exactly the same way as the Welsh language constructs them.
Meanwhile, eight-year old Angharad Fenner, who speaks mainly English at home, works out whether to spend her plastic pound;3 on rocking chairs, lorries or robots.
"The problem is resources," says reception teacher Nia Wyn Williams, who was taught in Welsh.
"Most of the computer teaching packages are in English. It's such a shame.
When you have a problem that asks the child to add one to 39, the Welsh-speaking child has no idea what 39 is. In Welsh it's tri deg naw, three-10s-nine."
It's so logical: reception children at Ysgol Ro Wen in the Conwy valley work out how many toys to buy