Teachers need more training and better support to deal with a rapid growth in the number of children of migrant workers, according to a new report by Estyn, the schools inspectorate.
Last year, the number of pupils from the 12 newest EU member states attending schools in Wales stood at an all-time high of 1,915, an increase of 40 per cent on 2007.
The Assembly government gave #163;9.6 million to local authorities through the Minority Ethnic Achievement Grant (MEAG) to support these pupils in 2008.
But while inspectors found that many schools have good arrangements in place to meet their learning needs, they said that generally there is insufficient good-quality training available for teachers and support staff.
Only a few local authorities use specialist support staff to help mainstream teachers.
The report says that the children of migrant workers joining classes adds significantly to teachers' planning and preparation workloads. And the situation is often made worse because most arrive unannounced, giving staff little time to prepare.
While most of the children are punctual and attend school regularly, a significant number take extended holidays to their home countries, and some younger children are often absent because school is not seen as a priority. This can impact on attendance figures in some local authorities.
The best support for children of migrant workers is provided in Welsh cities, and while most schools have purchased good quality resources to help them there is a general lack of suitable materials for specific subjects, the report says.
Estyn says the Assembly government should commission further research on the impact on schools of the rise in the number of children of migrant workers.
The government should also share information about where migrant families are settling to help schools put in place the right support, and share best practice across Wales.
Estyn called on schools to examine what impact the increasing numbers of children of migrant workers will have on them, and to consider further training for their staff.
They should also exploit the opportunity to help other pupils examine different cultures, it said.
Local authorities need to train more multilingual assistants to support pupils, and to make sure that MEAG resources are being shared fairly among schools.
However, NAHT Cymru cast doubt on the number of migrant worker children in Welsh schools quoted by Estyn.
In its report, Estyn conceded that it can be difficult to record accurately the numbers of children given population fluctuations.
Anna Brychan, director of NAHT Cymru, said: "There is a strong feeling among our members that these figures underestimate the real population and that this in turn makes it difficult to provide sufficient support services.
"We need more support to make sure that we can help them reach their potential - more training for teachers, more bilingual support staff and more language specialists, in particular. We know that these children can and do make a significantly positive contribution to Welsh life culturally and, in the future, economical."