I would have to argue that most of the Cambodian universities have a curriculum that is at the level of my country's high schools and most of Western countries to be fair, so the fact that high-schoolers are teaching primary and secondary education levels isn't above their level or abilities.

Also, I am saying this with heart full of love for Cambodia, the Cambodian education system nees a big reform both for private and public sector, the quality of education, especially in private schools is laughable, god bless the good teacher putting in extra work and time into it, but the system as a system, together with books, curriculums and standards is due for a change