Although I don't have the experience of having students in and out of classes so soon, I would suggest finding out how much grammar they know in their own language to start with. I believe a 6th grade education ensures that they have studied the sentence structure and that you can make the connection between relative clauses and pronouns in English and in their language.
New students arrive and leave every two weeks.
I can't teach grammar in any meaningful way. I've been struggling with this for 6 years. Our identities are very much part of our pedagogy. That is the question I asked and clarified.