I think 他在学校，因为他是老师 should be fine too.
My app marked that answer as acceptable
Agreed, this should be added as an acceptable translation.
I answered so too (;_;)
Did you try it? That was my correct answer.
It has been added - that's what i put and it was correct
What's the use of 所以 here？
It means so. Unlike English where you'd use either because or so, it's common in Chinese to say because x, so y
Because ...., So ...... .
It's equivalent to "so" or "therefore" - "[Because] he is a teacher, therefore/so he is at school" would be a more literal translation of the "correct" answer
It's like "that's why"
FYI in China:
We like to use 因为&所以 in the same sentence.
The current English translation is wrong. "因为他是老师，所以他在学校" = Because he is a teacher, he is at the school.
This is actually a good translation too.
Because of the range of different ways this question can be presented, it's not clear what you are saying is wrong. Your version is possible, but the English version without 'the' that I got is presented with is much more natural.
Can't I omit the second 他 ?
Agreed. The second "ta" is redundant
i take it because so is a type of sentence structure?
Would you not use "yi wei laoshi" (which is not offered as a choice)?