Re: High School Years Issue
Are you going all the way to graduation where you get the graduation ceremony or are you graduating your sims early? In my game, I found that if you graduate early, you keep the same NPCs in high school the next time. But if you keep your sim in high school until they reach the end of the teen life stage or age them up early with a cake, then you get the graduation ceremony with its graduation pic and that one or two of the NPC sims will graduate with you at the ceremony and become young adults. The NPC aging setting doesn't matter in this case. That doesn't hold true though for the teens used as NPCs like Molly, Ash, Kevin, etc. if they are marked as played or any other teen you have in a played household.
Hope this helps.