If you put “there” instead of “their”, and you fixed this, I don’t see this as an issue. The school is likely not concerned about the spelling of an in-class assignment because if you did it on a computer, it would have been spell-checked. If you start erasing sentences and putting in new sentences, I think that is an issue. You can always go back to the teacher and ask them if you spelled everything correctly, would the grade have been different? If it would not have been then you can feel confident that updating the spelling is not dishonest.