To put it simply one of my players can't make it to the session but there's a problem. During the last session he broke into a magic school and got his ass beat and captured. Normally I would just say whatever and jump back in time to the event when he gets back, but we can't just pretend that he's tied up (haha) for the next session because the party is leaving for an expedition to the mountains and that's going to take a while. Essentially, I want this event to hold meaning but depending on what I do with him changes stuff. If I have him with the party they know that he's fine and they don't need to worry and there is no tension when he gets back (also a couple people in the party found out what he did and went to the school to solve the problem peacefully) and if I don't have him with the party then I kind of doubt they will make it through the necessary kobold dungeon. What should I do?

Sounds like a quick rescue mission is in order.

The player doesn't have to be around for the party to rescue him. =)