from a KY teacher:
Cummins (1981) view is that assimilation of immigrant students is facilitated by maximizing experiences and practice with English if students are to be successful in school. This view, according to Cummins is grounded in a perspective on language dynamics that he calls separate underlying proficiency. The SUP assumes that the two languages operate independently. So, no transfer occurs between them. With this perspective, providing CLD students with resources, instruction, or literacy development in their native language would be a futile effort, increasing English language exposure is the path to English language development and school success. This interference hypothesis holds that ongoing development in the first language so interferes with second language learning that effort should not be wasted in either native language support or ongoing development in the first language. (Murry, 2005)
Cummins argues that although the two languages may seem separate on the surface, they are actually quite interdependent at the deeper level of cognitive functions. It is well established that students who learn to read and write in their first language are able to readily transfer those abilities to a second language. This transfer hypothesis is equally valid in other subject domains, including math and science.