Is the school being permitted to work on Saturday and Sunday?
Will the students come and attend class on such weekly off days? Naturally, no. Then why should the teachers come on Sunday? Not only are the teachers paid monthly, but every employee is paid monthly and all get a weekly off at least on one day after 6 days of working.
How many hours a person works is immaterial
A teacher attends a class for 4 hours a day, but that does not mean that he/she works only for 4 hours. If anybody says that a teacher works only for 4 hours, then ask him to take a session without preparation. He should take at least one hour to prepare for a session, and that means without being mentally and physically engaged for 8 hours, a person cannot take 4-hour sessions.
Therefore, he has every right to get at least one weekly off. He can approach the appropriate authorities if he is denied a weekly off.