As per my understanding, Shatin and Ma On Shan are in different school buses.
Ma On Shan's one would go to Sai Kung too.
So, the first station would be early, around 6:20am.
But, the detailed routes for the coming year is not fixed yet. I also waited for the further information on 9/7 ~ 10/7.
If you want to know the schedule for this year, you can phone to the school bus company which the tel no. is shown in the web of TSL.