Using such terms for adults comes as a bit odd, especially for fellow students to employ such terms, why the need for hierarchy when they are all under grads regardless of their age basically.
I don't think LiHuan has EVER called YuYang by his name throughout the entire series so far. I've seen some call Yuyang "学长" which is sort of something you refer to your upperclassman as. Of course, it's to show respect but you don't always use it especially to someone you're close to (I'm not entirely sure since I don't go to a Chinese school even though I live in a Chinese country). Lin Xiang (in Guang Xiang) also calls Yuyang's name as it is I think, because they were pretty close.
LiHuan's a pretty straightforward person who speaks little, so I think he's not the type to call Yuyang with a honorific.... Honestly, we won't be able to tell until the next chapter!
If anything, it's interesting to learn that such honorifics still exist past the mandatory education ages. Here, it would be expected that everyone be it student or tutor is just addressed by their first name, unless they specify otherwise. I guess it makes it easier though to just call people by teacher/professor/senior/junior etc if you have forgotten their name and don't feel confident enough to admit this to them though ha.
But on the scheme of things, and relating to the apparent cultural norms, LiHuan and Yuyang are still relatively unknown to each other, maybe they have known each other only for a few weeks by this point so I would assume calling Yuyang by his first name would be too familiar, even if Lihuan is generally more plain speaking.
I kind of saw that in chinese webtoons characters sometimes tend to call out to each other underclassman, upperclassman, classmate and so on... But is that a must? What I'm getting at, would LiHuan call out to YuYang without a honorific of some sort since YuYang is an upperclassman, or is that unneccessary in China and he can plainly say his name?