Mission details and observing eyes
In the first place the rewards were too generous! A spell slot was simply a chance to choose a new spell from the tower, given the spell module a mage could learn the spell in some time.
However a spell slot costs 20 merit points, he heard that a True mage could earn that much in a year and a half doing regular missions.
This was a very generous reward for apprentices like them. Not to mention the addition of a low-level enchanted rob.
This was enough to tell them just how irregular it was to assign this mission to two apprentices. The rewards or enough for true mages to covet this mission.
Senior Ron, "the crown Prince must travel in absolute secrecy, the fact that he left the capital must be kept under wraps at all costs. You will each take no more than two more apprentices. You will disguise yourselves as mercenaries going to the capital for work."