I would say that both are important. I agree that it is often difficult to learn the content of the material when it is in a foreign language. However, immersion is often the most effective way to learn a new language and a new culture. Often when a student moves to a new country, the schools do not have the resources to teach all the classes in their native language. I know that non-English speaking students in the US are put into the same classes with everyone else. However, they also take a special course which helps them learn English, understand their course work, and make other transitions. The teacher in the special class usually speaks their native language.
In America, I believe the English language should be most important to either native or second-language students. However, I see no reason why a student whose native language is not English should not also take his first language as well. For example, many younger native Spanish-speaking students (middle schoolers, for example) may not have a complete command and understanding of their language; while English should be the first priority, taking a Spanish class would also be beneficial to them.
Is this in America or in some other country? In the US, I think that immersion is better. I think that it is a better way to ensure that all students will actually learn English, which they really need in order to get ahead in American society. If we're talking about a country where English is not the majority language, then only those students who want to should take English classes.