A truant is a student who is out of school without permission. If you skip school to go shopping with your friends, you are a truant.
A person can be a truant, and behavior can be described as truant too, since it's both a noun and an adjective. Your teacher might call you a truant if you skip school and if you didn't show up at your donut shop job, you would also be a truant donut slinger. In fact, anyone who's unexpectedly absent can be described as truant. The Old French truant describes a beggar, especially one who lives this way by choice.