Why does school food taste bad?

School food is mass-produced and frozen. The lunch ladies put it in an oven and heat it up. The reason your Pizza is greasy is because that's the water/oil left over from where it used to be frozen. It's gross.

I understand that the school pays for this stuff, and some people may not get another meal, and my heart goes out to them. However, the fact still remains that the food tastes disgusting.