Warm the cooked macaroni and cheese in the microwave, stirring once or twice to heat it through. Once hot, add the shredded cheddar to the bowl and toss to combine. If all you have is sliced cheese, add ½ to 1 slice of cheese on top of the non-buttered side of the bread.
Assemble:
Divide the warmed mac and cheese on top of two slices of bread. Close the sandwiches, buttered side out.
Grill:
Heat a large non-stick pan over medium heat. Once hot, place the two grilled cheese sandwiches on the pan for about 2 minutes until it's golden and crispy. Flip the sandwiches over and grill the second side.
Remove the sandwiches from the pan onto a cutting board or plate, slice in half, and serve.
Pack for school:
If packing for school, wait for the sandwich to cool to room temperature and place it inside a hard lunch container. Keep lunch sides separate and pack everything in a lunch bag with an ice pack, if necessary.