People who go to the same school and are in the same class or grade.
Schoolmates are usually friends or at least know each other because they see each other every day at school.
Examples:
My schoolmates and I play together at recess.
I eat lunch with my schoolmates every day.
She sits with her schoolmates on the bus.
The schoolmates all work together on the project.
We are all schoolmates because we go to the same school.
The schoolmates have a big party together.