Why did Via snap at her mother when she got home from school?

Via snapped at her mother because she was angry about having received a lot of homework on the first day of school?

"She went straight home. We have a lot of homework."
"On your first day?"
"Yes, on our first day!" I yelled, which completely surprised Mom. But before she could say anything, I said: "School was fine. It's really big, though. The kids seem nice." I wanted to give her enough information so she wouldn't feel the need to ask me more. "How was Auggie's first day of school?"
Mom hesitated, her eyebrows still high up on her forehead from when I'd snapped at her a second earlier. "Okay," she said slowly, like she was letting out a breath.