Highschool graduation causing other teens to age up.

You're welcome, @GymeaLillie.
I can totally understand how this issue is pure frustration for rotational players.
I had to go back to an older save when it first happened in my game. There also is no warning at all.
Now that I think of it, skipping the graduation ceremony might be a workaround for Vanilla players - if you are willing to sacrifice the graduation event, that is.