England's first bilingual state school opened to acclaim in September but has had mixed fortunes in its first term.
Learning languages is becoming more popular in primary schools
By 2012, Wix School in Battersea, south London, aims to teach lessons in every primary year in French and English.
Six weeks after its first 14 reception pupils started, inspectors highlighted "some underachievement".
They said the project was enriching and innovative but some parents were not convinced of its benefits. Wandsworth Council said it was a success story.
The project is run with the Lycee Charles de Gaulle, an independent French school in London, with which Wix School shares a site.
Both schools admitted 14 pupils to the bilingual reception class and this will be repeated every year to create a "bilingual stream" at the Wix School, alongside classes taught solely in English.
Lessons follow the national curriculum but all subjects are studied in both languages.
The inspection by schools watchdog, Ofsted, was carried out in mid-October.
Inspectors praised the bilingual stream as an "innovative example of creativity" which had enriched its curriculum, extended community provision and improved its resources.
But they pointed out that teachers had not evaluated the impact on the rest of the school.
It also stated there had been some underachievement but the school was well aware of it and had begun to take "early and robust action".
The school declined to comment but a spokesman for Wandsworth Council said inspectors had only observed the beginnings of this "fantastically ambitious" project.
He said the school was leading the way by taking on a really innovative curriculum and it could not have got a better Ofsted report.
The project has the support of Wandsworth Council and the French Embassy.
On Thursday an interim report from Lord Dearing, commissioned by the government, backed the government's policy of encouraging language lessons in primary schools.
"For languages, the earlier the better," he said.
"We like the way they are being taught in primaries as they are introduced
through cross-curricular work, and the way they draw on the young children's
sense of fun.
"We propose that they should be embedded in the primary curriculum."