"nontraditional student" means in a college or university: a student who does not enroll immediately after graduating high school, works full-time or part-time during their enrollment, or is financially independent. It is one of many casual words teens pick up from friends, social media, music or sport, and on its own it carries no particular warning. If you hear your teen use it, it is fair to ask what they mean by it in their friend group, since meanings drift quickly. Asking out of curiosity rather than alarm tends to keep the conversation open and useful.