OK, so here's a way out: since there's going to be 2-3 years at the start of the saga before the teachers are ready to actually teach, Ancel can continue his training and try to squeeze in extra seasons (at the cost of his own development). When it's time to train students, he administers a slightly early Guantlet for his apprentice. After all, 15 year apprenticeships are merely traditional, not required by Hermetic Law. (c.f. Bjornaer who consider apprentices full members once they take on their Heartshape, or that Flambeau apprentice during the Schism War who was considered Gauntleted after 7 years after defeating some Diedne in battle) Trianoman Gauntlets are also straightforward -- send the apprentice to another Trianoma at a distant place to prove they can manage on their own. All she has to do is get there, and she's passed. That also gets her out of the school; we'll just assume she settles in another Tribunal. Perhaps back in Stonehenge, where she would've spent most of her apprenticeship.
It's a matter of Bonisagus pride to have a trained apprentice, so Ancel would prefer to see her on her way, even if it means breaking tradition. But given that he's at this school, he's clearly not averse to breaking tradition. Additionally, as a magus trained in the Rhine Tribunal, it also means he'd be Master status. (This doesn't mean anything outside the Rhine, but some habits die hard.)