Same for me my grade is D and talking with saeko at 17 dosent give me the option to enroll, my every save has had this same issue
At first it should be available only KAWAKITA tutor. u can start from 10pm-10am on Mon-Fri.(yup you have to skip class but it worth it.) At that point just study at school didn't improve your grade that much anyway. While tutor increase your grade faster. When your grade improved to B you can shift your gaze to Ami.
I did everything possible already. but how to pass this? I got to the end of every characters except this quest of Ema.
This is a repetitive quest, Emma's story is over. If he expands the story, it will also solve that problem that annoys a lot of people, I see.
Actually Saeko's apartment quest should be after you move to apartment with Remu. So you should progress with Remu path until you move with her to the apartment first. if you moved with Remu it will have the icon to go into the apartment lobby. and Saeko live in the same apartment.
the tutor reward event is quite random but all of them you must have the session before dark(the sky at the street is dark) so you can get reward. and it will appear while in the session you can ask for reward from her. if you have session afterdark the ask for reward won't show up(mostly as I remembered).