While you posted this problem in two separate threads, I'll respond here.
You are emphasizing that his brother is younger, but you haven't indicated by how much (also, what constitutes "almost finished"?).
I don't typically use boarding schools, but I believe there are sometimes glitches to the student aging up properly. Is it a gamebreaker if the sim comes home later?
What happens when you resetsim the sim in question and/or resetsim * ?