Placement test for large groups.
 

I am a North American living in Bogota Colombia, working for a customer service call center. Because I´m a "gringo", my boss made me an English instructor. I´ve never taught ESL before, so I´m going to start a certification course A.S.A.P. I know I have no business currently teaching ESL, but its happening, and I have no choice, so moving on. I was given the task of coming up with a level scale and a placement test to administer to 400+ agents in order to determine their level of English. My question is, what level scale should I use, and is there a free placement test on the web somewhere that could fit my needs? Any help would be greatly appreciated.