Two things...
Maybe someone can explain.
1. Red car test problem. If you send something back 1 minute in time, it will never catch up to you. The reason this worked in Back to the Future is because they sent the car 1 minute into the future and they caught up with it.
2. What happened to all their other duplicates? For instance, the scene when the kid is trying to pass his verbal test. He keeps going back to change it, but then there would be like 5 of him all showing up there at the same time. Going back in time doesn't mean you automatically replace the other 'you' that already went back in time.