Please don't bring up needless speculation on why Coco is graduating unless it's from her own mouth. All we know is that she would like to do her own thing and that we should respect her wishes. Pointing fingers/ flinging out accusations just adds salt to the wound that Coco herself would like to close up.
We should see her off with a smile and good wishes for her future endeavors. It's what our Kaichou wants :)
If when I die the higher being said to me that all the human civilization development were made just so "Adachi to Shimamura" could be created at one point...
^^ Eh, I wouldn't really call this a crackship. While there isn't very much Tenshi x Yukari here there's a good amount of fanart on Pixiv and Danbooru, and they have at least interacted with each other in canon.