My gosh my friend, we truly need more teachers like you in the world.... The kind that really care, and try to make a difference despite having to work in a system that you know most of the time doesn't work!

In Brazil, we have the same problem... Public schools are neglected severely. And even the private schools only teach mediocre English classes. My wife teaches English at a private school and their curriculum is just so disorganised, without much support from coordination. She does her best but she faces the same problem as you: Half the students are on a muuuuch lower level, and the other half (who have gone to specialised English course schools) get bored easily when she tries to address the lower-level students.

It's an on-going mission!