truant
A student who skips school without permission or good reason.
A truant is a student who stays away from school without permission or a good reason. When kids are truant, they're not sick or on a family trip: they're simply skipping school, often without their parents knowing.
Being truant is different from missing school with permission. If you stay home with the flu, you're absent but not truant. If you leave campus during lunch and don't come back, you're being truant. Schools track truancy carefully because students who frequently skip school fall behind in their learning and miss important opportunities.
The word can also describe the act itself: you might say someone is playing truant or is truant from their responsibilities. A police officer who's supposed to be on patrol but instead goes to the movies could be called truant from duty.
Historically, cities had truant officers who would find students skipping school and bring them back. Today, schools work with families when students become truant, since there's usually an underlying reason, like trouble at home, bullying, or falling so far behind that school feels overwhelming.