All the sentences in this lesson that use "mkuu wa shule" always translate as "a/the head of school". This seems unnatural to my (American) English. We would probably say "a / the head of the school" or "principal". I suppose it could also be headmaster, or head teacher.
But I'm not sure - maybe in Tanzania, "head of school" is the common English translation, just like "principal" is probably most common in the USA.