Home Secretary Theresa May was the guest of honour at St Piran's School in Maidenhead.
Joining staff and children at the Gringer Hill site she helped to officially declare open three new classrooms.
Following expansion work which started last summer the new learning space will help provide extra space for reception children.
Headteacher Jonathan Carroll said: "It has been a long time in the planning and we are absolutely thrilled with what has been built and what we have achieved.
"It gives a tremendous opportunity to the children."
During the opening ceremony, which took place on Friday, children at the school sung to the Maidenhead MP before she watched them enjoy their new classrooms.
The school has not said how much the expansion project cost.
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