Your schedulers don't know who to schedule for how much time, because you are not telling them.  Back when I was more concerned about being efficient, I prepared a list of clients and marked each of them with an A, B, C or D.  A meant 30 minutes, B was an hour, C was 90 minutes, and D was two hours.  This worked well, 90% of the time.  There were always a few complicated returns that turned into easy ones, and some easy ones that turned into complicated ones.  Often I knew about these changes, the previous year.  

Would you actually schedule 15-minute appointments?  I would be tempted to do what the doctors do.  Schedule 2 at the same time, figure one will show up early and the other might be late.