For my after school classes, I have 5th and 6th grade combined classes along with 3rd and 4th grade combined classes. I'm trying to figure out the best materials or topics to teach both classes with, but it's hard since the students are drastically different in levels. For example in my 3rd and 4th grade class I have a 4th grader who's able to have complex conversations and can read full paragraphs and understand them while I also have third graders who pretty much know no English whatsoever. I've tried to find something in the middle such as teaching words that the upper level student may not know but the 3rd graders struggle. I have a similar situation with my 5th graders too. One has learned a lot of grammar and can understand English really well and is good at speaking while I have another who can't even read consonant and short vowel sounds. My goal for the after school classes is to try to boost students' reading and speaking levels while also ensuring that they're learning about fun and interesting topics.
My first bit of advice is to explain the situation to your head teacher, CT or whoever. See if you can re-arrange the students by level; each student has a brief one on one interview with you, in English, where you ask them basic questions to assess their English level. Score them and arrange your classes accordingly... you need to assert yourself to get things done here.
For the lessons, you're not going to be able to teach an effective middle-ground. I suggest presentations.
Split your class into groups of 3-5, with at least one strong student in each group and each group being comprised of students of relatively the same level. Elicit some presentation topics (or come up with your own, 'most popular mobile game in Korea', 'ghost story', 'urban legend', 'BTS', 'uniforms or casual clothes', 'favourite animal', 'healthy lifestyle' or whatever) Have each group prepare a presentation/skit/play where they talk about their topic. Give each group a dictionary appropriate to their level and some examples or expressions. Give the strongest student of each group the role of captain and the responsibility of delegating tasks to the other members.
Walk around from group to group aiding them as they prepare their presentation/skit. Makes some props available.
Basically, you're not going to find a middle-ground to teach, particularly with a teacher-centred approach. Switch to making your classes learner-centred, your weak students will learn a hell of a lot from their peers than from you and the stronger students will have an opportunity to use their skills to lead and teach.
Keep the presentations and the atmosphere relaxed, light, fun and goofy. The minute things get too serious they'll start clamming up.
