Multi-level students can be a great opportunity for learning from one another. Try to find students with mismatched levels who have similar interests. If they become friends, they will be challenged to speak English to communicate and discuss their interests. Especially if your class consists of students who speak different first languages, they will be forced to speak English to communicate. There is little more rewarding to an ESL teacher than seeing students speak English with one another outside of class.
Use the same material for the whole class. Change the assignment or task for the student, not the material itself. For example, if you are using a reading assignment, use same assignment for all the students, but put the students into advanced and beginner groups. Have lower-level students complete a simple task, such as identifying the parts of speech of the word. Have the more advanced students answer questions about content of the writing ( such as an article.) If using a video, put a movie or TV clip on mute and have the advanced students write full sentences of what they think the dialogue is. Have the lower-level students write the adjectives and nouns they see.
Use the diversity of the class to your advantage. Find the specific strengths and weaknesses of students. Most students themselves are "multi-level," meaning they are strong in one aspect of language but weak in another. If one student has trouble with oral communication but has a solid grasp of grammar, pair him with a student who speaks fluently and has a vast vocabulary but makes frequent grammar mistakes.
Switch between teaching to the bottom of the class and the top of the class. When you cater to the lowest level, you can allow the higher level students to review. Even if they have a solid grasp of English, review is always helpful for foreign language learners. Teaching to the top of the class will challenge everyone.
Find out what your students are interested in and integrate these topics into your lessons. Use humor and keep the class light-hearted, positive and enjoyable. The lower-level students may be intimidated, so reassure them that there is no pressure in your class and that they will not be compared to the highest-level learners. The higher-level students can get bored as well, so keep the content interesting, relevant or quirky in order to retain their interest.