Eric, I'd follow Dr. Z's advice on getting the students front and center in the classroom. This will give you an idea of each student's speaking ability.

I'd add this: create a journal for each class. Make notes of each student's proficiency, and highlight those students who have exceptional ability and those with only limited ability. This will help you personalize your lessons in the coming weeks and assign appropriate homework. You don't want to bore the students 'in the know', and you don't want to blow away the students who are lacking. It's best to find a balance so the students can teach each other.

Good luck with your balancing act. I wish I were there with you.

E