Translation:We can go home early because there is no teacher.
Can this also be used as "I can go home early because there is no teacher" or is it clear that the subject is we
Yes and no.
Yes you can use "I" instead of "we".
No, it's not clear because this is a sentence without context. Given more context, you would know which pronoun was correct.
Wouldn't we say: We can go home early because the teacher didn't show up. ? OR, We can go home early because the teacher isn't there. ?
First one, I would say no because not showing up is like saying the teacher didn't come, which is a different verb (선생님이 오지 않았기 때문에 ...).
Second one is OK but the current Duolingo translation is the most accurate.