Looking for elementary and secondary schools

Hi everyone!
We are Canadian-Burundian moving to Kigali for two years next August; I will conduct some research there. We have 4 children aged 5, 6, 11 and 15. Would you have any recommendations about schools (elementary and secondary, public and private), and any experience on how schools decide in which grade to enroll your kids? Do they have any exams to pass?  We are not rich expats and we can't afford the tuition fees of many of the international schools I found on internet... your help, experience on looking for a school and recommendations would definitely give us a hand on that!

All the best,

Genevieve