The teacher in charge of a class (homeroom teacher), responsible for class management and student guidance in schools.
新学期が始まり、担任の先生が自己紹介をした。
The new term started and the homeroom teacher introduced themselves.
何かあれば担任の先生に相談してください。
If anything comes up, please consult your homeroom teacher.
担任の先生が行事の担当になって、準備で忙しい。
The homeroom teacher is in charge of the event, so they're busy with preparations.