For Arbor's good end, you must meet her when you have Arithmancy class and then meet her again at lunch, choose the suitable option when interacting with her. Once you finish your two interactions with her (yes, there are only two), die. Eat shrooms or try any other possible ways to die and her route will proceed. Choose the option "you have sth to do" (I don't remember its full text) and the story will begin.
For Barnaby's bad end, keep bullying him and NOT even try to increase his favorability. Make sure that you have gone through all five meetings with him.
Hope this help!