Bobby Brady: Mom always says not to play ball in the house.
Jan Brady: Well, all day long at school I hear how great Marcia is at this or how wonderful Marcia did that! Marcia, Marcia, Marcia.
Marcia Brady: Ooh! My nose.
Alice Nelson: If there's anything I can't stand, it's a perfect kid. And SIX of 'em, yecch.
Marcia Brady: It's so beneficial for me to be away from those children in junior high and to be with people of my own mature growth. I'm looking forward to the intellectual stimulation. Nice meeting you boys. Bye.
Answer: There were several episodes throughout the run of the show where Marcia was infatuated with a guy she went to school with. This bracelet could have been a gift from her then-boyfriend; as such, she would not want to take it off, as a sign of loyalty. A stupid thing to do, of course, but it's not a mistake for a character simply to be dumb.
Matty Blast