Well for either Renwick or Luz, they couldn’t find any property near enough to the school that would allow them to plant a tree temporarily, since that would involve digging a huge fucking hole and then putting a tree there that can not be allowed to become the property of the owner of the land. Basically no one within a close enough radius of Luz’s chosen school would allow it - there were some closer options, but far enough out of range to be somewhat pointless.
Also keep in mind that Dryads are insanely protective of their trees. There were some odd rare situations where Luz could have planted his tree somewhere but didn’t trust the property owner and was basically being insanely paranoid, and didn’t feel comfortable leaving his tree where neighboring Dryads couldn’t keep an eye on it while he was gone.
Luz and Renwick come from the same general area and sort of knew OF each other since childhood, but they weren’t really acquainted. Their parent’s trees were far apart, their parents didn’t know each other, they kind of just didn’t have any reason to interact outside seeing each other occasionally when all the local Dryads gathered together and all the children were gathered together in a small crowd. So that’s when they met, but when they met for real, actually becoming friends and then later a couple, was when Luz went to medical school - but not AT medical school.
Renwick had an interest in making medicine for a long time and had started to apprentice under another Dryad to learn their folk remedies, but he’d also wished he could learn about human pharmacological medicine, but didn’t think he could do that. The injury his tree had sustained and its effect on him made him extremely cautious, which left him more acutely aware of being a bit stuck where he was.
While that was happening, Luz, separately, had decided he wanted to become a surgeon. He wanted to cut people open and save lives! (those were both important details of the dream. Surgery is kinda weird when you think about it) And he’d go to medical school and learn how to do it if it killed him! (and it nearly did) Suddenly nameless Dryad in the community number 321 became “That crazy willow kid who’s going to medical school”
Ren immediately sought Luz out when he heard about this, and would sit and wait by Luz’s tree every time Luz came home for a break from medical school so he could go on talking to him (and also read his textbooks). They hit it off famously.
Luz also paved the way for Renwick to go to medical school too. He grappled with the system to find a balance between needing to be there, needing to not keel over and needing them to not expel him because of the breaks. And he convinced Ren he could do it too, so by the time Renwick gave in and tried medical school too, they were better prepared for a Dryad with more delicate health who wasn’t as willing to flirt with death (if he’d ever been threatened with expulsion, he probably would have given up rather than go and talk to every possible person in the chain and attempt to charm them to death - which is what Luz did whenever he met a wall).
Renwick is an apothecary/pharmacist.