I'm mostly speculating but I just got Bugbear's end, so here's my best guess. Lmk if it works!
For Bugbear, make sure you have crypto and astro (ik you need crypto, afaik you don't need astro, just some other class besides crypto for him to hopefully talk to you in, astro just worked for me.)
Also make sure you have some bad student in you asap (someone else recommended 4) and try to talk to him as much as possible while still stacking str.
For in between events where you have minor interactions, I find him mostly in the garden, forest (at night), and I've gotten two unique interactions, one in the bathhouse and one getting checked up at the infirmary. (I know the bathhouse one is not req. to get his good end, and I don't think the infirmary one is either.) You can tell whether or not bugbear is in the forest based off of whether the trees have red faces. Early on in his route if he's there, they'll only have specks of red on them. (Red eyes, maybe?)
two events should trigger, before you get either conversation, bugbear should tackle you. This happened to me once in the forest and once in the garden, both while wandering (but I hear it can happen in the field too).
After that one, you should get another where bugbear asks you to have some fun with him, if you say yes it'll prompt you to confirm entrance to his route. (Saying no or "avoid"ing his route will lock you out I believe). The only time I can remember this convo being prompted for me was in the garden, but idk if that was by chance or if it has to happen there.
skip all classes until you hit 10 str (picking bellatrix can help out wit this). Once you do that, stop skipping astro. skip crypto until you get the convo in astro bc otherwise the convo abt his smile will initiate in crypto and you won't have enough time to initiate the convo abt bugbear before friday.
In astro, hopefully he'll talk to you. I asked him everything and the only time I refused to prod was about his family, I don't know what'll give you more points but I've heard you need to ask about his smile. At the end he'll ask if you'll be at the library.
take crypto next opportunity you have, a prompt should come up to ask the prof about bugbear, click it.
After that, you can trigger his library event. Mine triggered a day after the convo in astro while reading, so dw about time.
then, just hang out with him where you can until friday :D (fyi, trying to stop him in his ending or letting it happen doesn't change anything!)
I hope this helped, I'm sorry about all the uncertainties ;-; I've actually been trying to figure out how to get his bad end. (If anyone knows pls help) lmk if you need any more clarification!