"Youíre supposed to" seems dubious, interpreted either as "be scheduled" or "should". How can I interpret it?
W: Thatís very sweet of you, but I can take care of it. Why donít you do your homework?
M: Oh, Iíve already finished it.
W: Thatís great. You just sit there and tell me about your day.
M: Okay. Oh, can we talk later, Mom? I forgot to buy the tickets for the soccer game.
W: Right. Youíre supposed to take your brother to the soccer game this Saturday.
M: Yes. Iíll be back soon. It wonít take that long.
Use either "obligated", promised to", or, "this is part of a plan".