Could Mr Z be thought of as the protagonist? He is trying to encourage the kids in math and wants to avoid conflict. At the end, his interactions with Maura and Greg change how he thinks about change and advocating for oneself.

No, I do not believe that Mr. Z can be thought of as the protagonist in Lunch Money. He's a central and pivotal character for sure, but at no point would I feel comfortable saying that he is the main and central protagonist of this story. Greg and Maura are the protagonists. It might be an interesting argument to hear about which of those two characters would be the protagonist, but Mr. Z isn't as central to the story as the two kids. Mr. Z is important in that he settles arguments between the two kids, and he functions as a sounding board for their arguments, ideas, and business plans. He also gets them to think about things other than themselves. Finally, he does publicly support them in front of the school board. Despite all of those really important roles, Mr. Z is not the protagonist of this story because he is not a character that is driving events forward. He steers the kids, but they still have to be the people that ultimately take action about his ideas.