Dead giveaway
You see, this is where having the same looks is simply not enough to get by. You also have to be able to pull off whatever the thing is that you are trying to do. In this case, one twin knew how to write in cursive while the other did not. This is not the same as knowing how to do a math problem, this is something that the teacher will notice right away. And it looks like that is exactly what happened here when they tried to pull off the old switcheroo.