Leftovers need to be heated to 165 F. in order to be safe.
If they are to be held hot, they need to be held at least 140 F. for no more than 4 hours. The food can be heated back up to 165 F within 4 hours and then held again for another 4 hours at 140 F.
The two important things in food safty are the two things that cooks have the most control over: time and temperature.
By the way, what cooking school are you going to?