Some schools or mission organizations require experience and some do not. Mission schools are busy places and new teachers need to be able to stand on their own two feet. Previous teaching experience will contribute to the teacher's success. First year teachers in any situation find themselves grappling with classroom issues of academics, discipline, administration, and parents. This is compounded when the new teacher is in a new culture, perhaps in a different climate, perhaps is away from home for the first time.
Children of missionaries deserve the very best possible education they can obtain and much of this is related to teachers!