There's a guide link in the post from 13 days ago on a google docs you can use. It's a bit confusing at first. If it helps to simplify it, i basically did all of the quests, unlocked all diary entrees, touched the globe when i break into the headmasters office to make Arshem help save me later by bringing me to the headmaster.
Also, talk to him on every day you have the option to click on different places and save before your convos with him. That way you know what he likes and doesn't like. And then reload and pick the same options that make him happy. He also likes to be your sugar daddy, so on your date with him for his bday, pick the most expensive plate. Not doing so drops his affection for you. You can check their notes to see which choices also give you more or less heart ❤️ status with him.