React to my children’s bad temper

I feel tense when my son throws a fit because he doesn’t like what I say . I want to stay firm and hold into the schedule but he just cried and shouted when I told him that he should take his class earlier today than usual . He is 12 and he takes math class after school along with his sister . She was not feeling well today so I told him that he was going to take the class and his sister won’t. He didn’t like that and said that I favor his sister . I don’t know how to convince him or deal with him effectively. He seems irrational and disagreeable to me . I want to handle the situation properly without sabotaging our relationship.