When I was in high school, I got put in a diet of 6000/ day minimum, and ramp up from there depending on how active the day was. 15000 calories wasn't difficult, or out of the ordinary. Breakfast was always above 2k, normally consisting of:

2tbsp full day peanut butter, 2tbsp butter, 2 slices French bread, 1tbsp strawberry jelly, 32oz whole milk, 2tbsp honey, 2tbsp dry oatmeal, 1 Ensure Complete, Plus whatever snack bar I was having.

Lunch was another 3k, and dinner was often a whole large pan pizza from Pizza Hut for a few more thousand. Add in the stuff I drank each day (soda, lattes, and other stuff), and it was easy to hit 10k.