Thanks Danton and JustLurksHere. I was lucky to have a save file ready for a route flag, so I could quickly recover my lost progress for the 0 rumor route, though I'm stuck with no idea how to trigger the ending itself. And it allowed me to guess what locked me into the stalker route.
The game has 4 phases known to me (depends on your corruption points, affects which events you can view):
1. Yumina clothed without panties
2. Yumina in underwear
3. Yumina nude, shy (covering her privates)
4. Yumina nude, not shy
I'm pulling this from my memory:
* Bathhouse has a peeping event in all phases except #1, watching them unlocks the secret hot spring event if you inspect Yumina's computer.
* Pay attention to Yumina's team mates, if one is standing in a different place, there's likely an unlockable event. IIRC Miuna is easy to overlook, gotta talk to her while she's in the Gym entrance room after training but before changing your outfit?
* IIRC the art room events unlock depending on phase, at some point a new girl will appear in your classroom and ask you to become a model for the club. Accept and go to the club. Second event probably unlocks from a new NPC in school F3, last event unlocks when a new NPC appears in the art room.
* In the magazine shop (mall, north?) during phase 3 (?) you may see a shining point near the wall. Pick it up and go to Yumina's computer to unlock the small selfie event (leads nowhere, she chickened out... )
* I think tutored kid events unlock after you enter the helper route? (haven't unlocked any yet)
* NPCs and some events are random, the game rolls them at the beginning of the day, so save each time before going to bed.
* ? (there is an event that unlocks depending on Yumina's stress, afaik. I'm too tired right now to translate or test it properly)
* keep your stress low by clearing the exhibition part & exhibition games without getting caught. When it reaches 300~, it's game over.
* Stalker route is triggered if you talk to the stalker a lot. Talk to him each day and eventually Yumina will run away, afraid of him. This is probably the route flag and the game will automatically lead you into the ending after he catches you going into the exhibition part
* during phase 2, the eastern area has an exhibition game with a route flag warning (Yumina handing out flyers). I haven't tested this much, it's the route of the guy who appears at the very beginning in front of the gym. One ot the requirements for this route might be meeting the guy a few times, including a shiny point in the mall, north-eastern shop (phase 2?).
* during phase 3, the northern area has an exhibition game with a route flag warning (helper route)
* during phase 2, there's an event in the eastern area where Yumina will see a scary person and run away. In phase 3, return to the eastern area and follow the path below the entrance, there is an exhibition game with a route flag warning, starting it will immediately lock you into the delinquent route.
* raising your rumor points will lock you into the gym coach route, it progresses with each phase (because rumor points have a limit depending on phase). After you reach the point where he found something to blackmail Yumina with (phase 4), start the exhibition part, immediately walk back into the toilet, use the toilet and then get seen 3 times in this same area. (I haven't tested what to do next yet)
...
I still don't know how to unlock the true love route (I think one of the NPCs in the school is giving you hints: 4F, the one between bookshelves) or how to trigger the 0 rumor ending. Maybe they're the same thing, maybe it unlocks in the same way as the stalker route.
EDIT:
* for the route flag in phase 2: Do that exhibition event, afterwards when you're collecting shining items during the exhibition part, make sure the final item is in the eastern area. The NPC will witness you doing it. Repeat it again and you're locked into the route.
* about the helper route: you can watch all tutoring events after Yumina gets attacked and before getting the last 2 helper events.