When Kat is introduced in her new school at Friendship, Casper ties everyone's shoelaces together. But when Amber stands up to say that they can have the Halloween disco on her boathouse she doesn't fall over ! But then when the bells rings she does!
I think it's just because you don't necessarily move one foot forward when you stand up from a desk. You can usually just move both feet at the same time, THEN put one foot forward when you're walking. Then again, I could be wrong.