One incorrect assumption is that bilingualism leads to language communication disorders (e.g. language delay). Research does not attribute such disorders to bilingualism. Rather, such beliefs derive from prejudice and ignorance of linguistic and cognitive research. The communicative differences of bilingual children must be distinguished from communicative disorders. The failure to make this important distinction partly occurs because basic mistakes in assessment and categorization are sometimes made.
A bilingual child is often assessed in their weaker, second language. Hence, both language development and general cognitive development are measured inaccurately. For example, in the US and the UK, immigrant children are sometimes assessed through the medium of English and on their English proficiency. Their level of language competence in Spanish, Vietnamese, Hmong, Korean, Cantonese, Turkish, Talagog, Bengali or Panjabi, for example, is ignored.
The result is that such children can be classed as having a ‘language disability’ and perhaps a ‘learning disability’. Instead of being seen as developing bilinguals (i.e. children with a good command of their first language who are in the process of acquiring a second, majority language), they may be classed as of ‘limited English proficiency’ (LEP in the US), or even as having general difficulties with learning. Their below-average test scores in the second language (e.g. English) are wrongly defined as a ‘deficit’ or ‘disability’ that can be remedied by some form of special education.
…Bilingual children are often over-represented in special needs education, and this is much due to biased assessment practices. Assessment can result in both cultural and linguistic bias, in the testing and the tester, in interpretation, discounting and omission.
Above excerpt is from Foundations of Bilingual Education and Bilingualism, by Colin Baker, pp. 349 & 353. Multilingual Matters, 2006.