Yep, I've done this and it wasn't easy! The best way I could figure it out was to mentor the more junior students with ones on the higher level and given that the higher level students were helping the lower level, they'd then be working harder and using more vocabulary etc. I'd also give them credits for their assistance etc as it's always nice to recognised for being helpful.
The real problem comes at exam time I guess, when you've basically taught the same content to both levels, but have to make 2 exams up, one for each level.
What I did there was to make the questions a little harder to understand and answer for the upper level and made it the same for the lower level without the why questions etc.
It all depends on which levels you're talking about too. High school and College students, for example, should already have a decent enough proficiency to be able to take it all in their strides and realise what's happening. It always pays to explain what's happening to your students beforehand just in case though. I know of some students who told their parents that I was concentrating too much on the lower levels and not enough on the higher level. It's a hard thing to do both, but it can be done. There isn't really any "right" way to do it so just do the best you can!
Good luck!
