What is the guideline for instructional placement of ESL students?

Which students should be placed with a certified ESL teacher? High functioning, middle functioning or low? I'm not ESL certified & had high students who did extremely well this year. The low students were placed with another non-certified ESL teacher while the middle functioning were placed with the ESL certified teacher. Next year, they are placing the high students with the certified teacher. This doesn't make sense to me. What are the guidelines?