If you are forced to work on a Sunday, does the school need to pay you 1.5X or anything like that?
Also, the next 6 days are work days after tomorrow so is working 7 days in a row legal?
No.
Yes.
Short answer: talk to the labor department.
Long answer: too many rule changes in recent years, don’t know if your school is a buxiban or not, too tired to look into it… 
(sorry) But if the day in question does count as a day-on-which-leave-should-occur (which would be determined in relation to the rest of your work schedule, not which day of the week it is, i.e. “Sunday” could be any day), the rate should be 2x not 1.5x iirc.
Did you already check Labor Standards Act, especially its Article 24?
Anyone who has internet access can read the law, and it is translated in English, though it may be not a perfect translation.