Good luck!! I don't know how you are going to handle such an enormous range of ages and skills.
I work in a large American adult education ESL program with 8 levels. Any time we get a new student, that person goes through assessment tests in reading, speaking and writing. The results are used to place the student, since age and grade are only considered for children. We always use the CASAS reading tests, and they are really useful. Maybe you can buy some CASAS materials for use in your new location. Here is the CASAS website:
https://www.casas.org/home/index.cfm
Are you going to work for a school, operate your own school, or tutor students? If you are going to work in a school, which grades are you going to teach?