I did some testing by installing Sims 4 with only HSY and growing together on my Macbook (no mods), and a new brand teen in a brand new world was created.
On game start, I immediately aged the teen into young adult.
The result is similar to my original game, and there was no Graduate from High School milestone, but Aged Up milestone (in Young Adult) was present.
I saw no Graduation Ceremony in my calendar, and no diploma in my household inventory right after aging up.
The next day (Monday), I received the diploma in my household inventory after collecting mail, and calendar invite from Grad Ceremony showed for the following Sunday from 6 to 11:30 PM, but there was still no Graduate from High School milestone. (For those with "Did not receive calendar invite for Grad Ceremony", perhaps just wait until the next day until you receive you diploma).
I sped though the week and into next Sunday for Grad Ceremony, and I got the reminder to join Ceremony. I attended the ceremony and got my ceremony diploma + photo, but still no milestone.
So, despite the many website that claims there's a Graduate from High School milestone, I was unable to get it.
This is my theory:
As soon as I aged up my teen sim, they are already a young adult, but the Graduate from High School milestone restricted to a teen, and the game is unable to add the milestone to my sim. This is probably an order of operation problem. Please fix!
As per my prior post, the milestones for Drop Out of High School and Early Graduation from High School worked fine because the sim would still be a teen when they left high school.
