Well I'm not going to answer this in-depth, but the first thing that pops into my mind is -- get everybody speaking and listening, speaking and listening ... Speak English yourself as much as possible, so that the lower-level students get exposed to it. They need to get a "feel" for the language. You don't want the higher-level students to be slowed down and held back by the lower-level students. You will need to compromise, in every lesson. Maybe the higher-level students can do a more advanced version of the activity, and you can give more of your time helping the lower-level students with a simpler version. But please remember to keep the class together as 1 unit, and try not to seperate the lower-level students by putting them all in a group -- mix everybody up! The lower-level students will learn a lot from the higher-level students.