Other nations don't find it difficult. You just throw money at the problem, and the problem goes away. Like most problems.

Deciding that we need to serve food at a minimum of cost with a minimum of staff who is minimally trained according to a minimalistic nutritional guidelines, and charge children for the privilege of choosing to eat, and you aren't getting a feast full of fresh produce.

Japan is a decent model in making meals more communal and spreading the labor requirements around to students so that staff can focus on back of house work, but it starts with a higher budget basis to start with, makes meals mandatory, and provides significant subsidies.