When Quentin enters the washroom at the party, the shower curtains are already open, but then later on you see Lacey opening the shower curtains.
In the beginning of the movie, it shows flashbacks of Margo crawling out her bedroom window and Quentin seeing her from his; the houses were next to each other by the way the windows were facing. Later in the movie, we see Margo go to her house which is now across the street from Quentin's.
When young Margo and Quentin find the dead man, his face is turned away from the street toward the pond in the park. In the next shot when Margo gets up close, his face is turned towards her and the street.
Q says (in voice over) that Margo never made it to school "that day, or the next, or the next." He tells the detective that the last time he saw her was Wednesday night for a few minutes. That would make the first day she missed school Thursday, followed by Friday and on the third day - Saturday - these is no school.