Why does Via go by Olivia at school?

Via decided to use her full name at school, but to remain Via to her family. For Via (Olivia), this gave her, her own identity.

What I always loved most about middle school was that it was separate and different from home. I could go there and be Olivia Pullman—not Via, which is my name at home.

Because Olivia is her full name, and only her family calling her Via. So, it is worth calling her only to people close to her.