That does sound tricky, my school is divided for 1st and 2nd to have their own afterschool classes and 3-6 (the 6th grader do not come as I see them 3 times already in a week).
I could have the 1st and 2nd together and I think it would workl, but yeah I agree that to have 1-4 together seems like very different attention span for the different ages.

Maybe take a look at the variables and see if you can rearrange things? "Make" it fit a designed lesson plan to facilitate further educational purpose and present that politely to the person who can make changes. Maybe first suggest it to your co-teacher, if s/he doesn't seem sure, take that opportunity to politely say you will ask the principal and see what they think (I had a good relationship with my principal). Although if you present it confident and look like you know what you're talking about (it helps if you do ), past staff have welcomed something that was suppose to help the student. This of course relies also heavily on your relationship with your school's staff, but I think is possible, as after a few years of teaching in public schools, they are usually chill with my approach and logic. The super super traditional Korean principal though... you may have to just bite your lip. This is only my 20 won though.



Maybe pair them with equal levels and have them get used to their team of mixed years?
(This takes some patience and effort to create and maintain)

I guess last resort, if merging English ability (tricky but possible to a limit) is proving too difficult (which I think this may be a contender, is because of the age difference being the cause in ability to learn more than whether the student has past English lessons) you could just focus on the majority's level and let the class weed out, but that would be my last resort.