Most of the characters are in Clay Jensen's (Dylan Minnette) and Jessica's (Alisha Boe) graduating class, and so for them, seasons 1 and 2 were junior year while and seasons 3 and 4 were senior year. That also explains why some characters, like Marcus (Steven Silver), Ryan (Tommy Dorfman), and Courtney (Michelle Selene Ang), left 13 Reasons Why as regulars after season 2. They graduated, and hopefully got very far away from Crestmont.