To what extent does bilingualism help in education? There has been a lot of debate over bilingualism in education. I have read a bit on the website of the National Association for Bilingual Education and some other links to read about bilingualism. However being an amateur in this field, I'd like to know what are the major works (if not conclusive) that support and dissuade against bilingualism in education especially in the context of cognitive science?
This is a phenomenom I was able to witness first hand when I used to work in an International School, where every student had to learn Japanese and English, as well as having their mother-tongue (some had several more languages as well).

In the paper "Bilingualism and cognitive development in relation to threshold theory" (Ricciardelli, 1992), it is stated that:


  This theory (threshold theory) maintains that there may be levels of linguistic proficiency which bilingual children must attain in order to avoid cognitive deficits and to allow the cognitive benefits.


Further, in "A Systematic Review and Meta-Analysis of the Cognitive Correlates of Bilingualism" (Adesope, 2010), some links between bilingualism and some increased cognitive abilities have been suggested from research.  From their abstract:


  Results indicate that bilingualism is reliably associated with several cognitive outcomes, including increased attentional control, working memory, metalinguistic awareness, and abstract and symbolic representation skills.


A link between bilingualism and how the bilingual student would tackle worded maths problems, in Cognitive Benefits and Costs of Bilingualism in Elementary School Students:
The Case of Mathematical Word Problems (Kempert et al. 2011).

An important point 


  bilinguals gained scores comparable to those of their monolingual peers on
  word problems that required attentional control skills although performing significantly below their
  monolingual classmates on ordinary word problems, suggesting that bilinguals have an advantage
  when it comes to attentional control.


However, there are potential cognitive costs in transferring knowledge from one language to another.

I hope this helps