Why Courtney and Ryan...?

My take, and maybe I'm not recalling every detail from seasons 2 and 3, is:
- Ryan is a year older than the others (as Bryce and Monty were), and is allegedly back from his sabbathical around Europe (he's most likely from a rich and privileged family, though we never get to meet his folks and possible siblings), just to say goodbye to his former accomplices/kind of friends. He's shown to be grieving Hannah, and there's the tie with the LH "justice-driven" gang.
- Courtney is definitely in everyone else's same class. I don't think anything like that was mentioned throughout his brief S3 appearance, but, being a top-grade student, probably on Ani's tier, there's a chance she started college a year before the other ones, or even got to to take a rest/travel throughout the second half-school year, maybe out of having gained enough credits to enter the college she wanted to (most likely an Ivy League one, since his parents look pretty wealthy). That would explain his total non-involvement in everything going on throughout S4 (a clarifying line would have been appreciated, though).
- The unveiling of Marcus' misdemeanors in both Hannah's suicide build-up and the Clubhouse sexual assaults clearly ruined his father's political career, and is safe to say the whole family moved elsewhere. Marcus was a dick up until the end of his arc, and, unlike most of the others driving Hannah to take her own life away, showed no sign of redemption. Therefore, quoting Chuck Schuldiner's Death, no "moment of clarity" towards Clay and the gang, he just disappeared from everyone's lives. As sometimes happens in real life, we just don't get to know what future he met: he either may, eventually, have got to face the shame his deeds bore, and even embarking a "be-better" path, or, once having been discharged from a legal standpoint, kept on being a total prick. Possibly even a worse one, if he got a load-free pass on what he did.