I think that is up for discussion. It would depend on the English level of the students, for one thing. My daughter was in a dual-language program since first grade that aimed for half-Spanish, half-English, and I think it was very successful. On the other hand, I’ve seem bilingual programs that neglected English altogether, and pulled my niece out of one when she was six.
But language classes ought to be taught totally in the target language, uncomfortable though it may be at first.