TPE C-ELL

I will be teaching an ELD class next year and I know the biggest challenge will be the range of ELLs I will have.  Some students speak little English while others were overlooked as ELLs last year because they seemed to speak English "enough to get by".  I have heard before that a student's ability to learn English well is reflected in their own native language proficiency, therefore teachers, "need to know who the students are and what their prior education experiences were like."  I'd like to know if this is as informal as asking them or if there is a more formal assessment.  I also am worried I might create curriculum that is too basic or elementary for high school students, so am interested in learning more about how to ensure that I am teaching them content standards.

The CAHSEE exam is the one exam that has made me want to commit to the ELL students.  I have seen how challenging and difficult the ELA test is for them.  There were 4 ELL students who did not receive their diplomas last year only because they did not pass the CAHSEE exam.  I want to know what we can do to help them pass this test.