The sentence is "諸星先生は元々住んでる星が違うし". This looks to me an idiomatic expression, but I cannot find this in dictionary.
Another interpretation I was thinking about is that the teacher in the above sentence "lives in another star/solar system", which is to say that it's just a completely different person (in character). It fits, but not sure if it is accurate; for one, living in another planet feels more accurate, and if so 星 should be short for 惑星?