The seat to Riley's left in school is open and then has a student in it with no time between the shots.
When the kids are in school there are books on the desk behind Riley but no student until Lucas comes in later.
Riley prys the door open to the subway she and Mya were on hen she fights for their friendship. Subway doors in New York cannot be opened like that once they're closed, especially by a 13 year old girl, and enough time had passed after they did that Riley wouldn't have had time to go back and try, as Mya would already be on her way to the next stop.
When Maya attempts to burn the homework with firework sparklers Corey Matthews take the homework from her. After that she ends up pointing the sparklers to a fire sprinkler head causing all the sprinklers in the classroom to activate. Fire sprinklers do not all go off at once.