Ey, show the teachers some respect. They put in their own money to get the pizza, money they earned. Teachers don't get paid well, be happy with what you get.
I'd be surprised if the money used to buy the pizza was the teacher's. It's likely part of the school’s budget if it had to guess. Regardless, they’d save a lot more money if they didn't do this type of stuff. It's always hyped up as a big reward or to show appreciation and then you get this… I don't think anyone is actually satisfied and they might even work less hard/slack off from then on if the reward isn't worth it to them. It's a bit demotivating.
Part of the reason I believe it to be part of the school’s budget that pays for the pizza is because they have to obtain permission to even do things like this to start with.
I have an aunt though that used to be a 4th grade teacher. I could probably get some insight into the actual answer over Easter.