I am not sure that the food in our state prisons are better than what is served in our schools.
I have eaten prison food and it is not all that great as the people that work in food service
are largely inmates and the quality of stuff that they purchase is not the greatest.
The exception might be the food they serve at the Federal Detention Facility out at the airport
but the food there is not cooked fresh every day. They cook the food 2 or 3 days of the week and
flash freeze it in food carts and the food is then reheated before being served to the detainees. 
Like a lot of things, it can come down to who is doing the cooking and what their skills are.