Catholic schools are struggling to cope with ballooning registers as migrant worker families flood Wales.
Workers from Poland, India and the Philippines top a list of foreign nationals wanting a faith school education for their children when they settle here.
But heads say the Assembly government is ill-prepared for the influx, and places are fast running out. They also claim schools cannot afford the language needs of new pupils who do not have English as their mother tongue.
Martin Price, vice chair of governors at St Richard Gwyn RC high school, Barry, Vale of Glamorgan, said: "Our school is over-subscribed. We aim to get Catholic children into the school but it's increasingly difficult."
At Bishop Hedley RC high in Merthyr Tydfil there are a "significant" number of Polish, Portuguese and Filipino pupils. Within five years the number of pupils with English as an additional language needs has grown by 36.
Headteacher Maureen Harris said she plans to improve language support by applying to the Assembly's new pound;16 million Raise fund for disadvantaged pupils.
EAL teaching support is usually funded from the Ethnic Minority Achievement Grant, which the Assembly increased from pound;4.5m to pound;5m this year. But schools do not become eligible for the grant until pupils appear on the school census every January.
At Our Lady St Michael's in Abergavenny 36 of the school's 240 children are from other countries. The school was receiving one day a week of language support from Newport-based Gwent Education Multi-ethnic Support Service (GEMSS), but demand has reduced it to a half-day.
Headteacher John Healey said: "GEMSS provides language support and staff training and they're helping out as best they can within budgetary constraints."
Tracey Pead, deputy service leader of GEMSS, said: "Lots of minorities will want their children to have a Catholic education. But nobody took into account the European Union accession states which are new languages to us.
We've had 247 from these states since April, but we won't get any funding for them until next April."
Roy Jefferies, head of St Mary's Catholic primary in Cardiff, has 38 children of other nationalities who speak 12 different languages. Since September 2005 the number of Polish pupils has increased from three to 12 at the 230-pupil school. He said: "The LEA has been very supportive, but I'm aware that once they increase the support, they have to go cap in hand to the Assembly."
Cannon Peter Collins, chairman of the schools commission in the archdiocese of Cardiff, said: "There's been an increase, particularly in some schools."
Reverend Edwin Counsell, education officer for the Church in Wales, said he had not seen the same changes in his schools. "Those who are coming in, particularly Poles, are joining the Catholic community and parents are making that choice, he said.
A spokesman for the Assembly said there were no figures available that show the impact of economic migration on schools.