You get the S.A.D through the library job. Without spoiling anything, you have to work until you're prompted to speak with Beth about something.
I have a 4 in all stats, maxed affinity with Beth (gone on a date), maxed library job, and still nothing
Try talking with her, there are more options!
Edit: I just checked, looks like it was only accessible at Beth level 4, I'll fix it now.
