To follow Rose into the aether, the headmaster will have to drop his body, the body he foreswore his ethical precepts to preserve, the body he fed with farm raised prana, the body that served him for over a century and a half. As a fully realized adept, the headmaster understands that he is not his body, that his true nature is unaffected by change, and even if they are cared for with a steady supply of life energy, the telomeres of his human body's cells can only be extended for so long.
The headmaster's top protege is already proficient in the art of conjuring and harvesting shroops. The succession at the school is well planned. There is no need for any theater around his passing - in fact many of the students are ready to track his movements and try to assist in the mission by using the power of the scrying basin. It isn't Rose that they want to track down, the headmaster is taking this extraordinary step because he feels that he is closer than ever to putting a stop to TNAP and absorbing Her power.
The ceremony is simple, the jump is risky. No one gets emotional. They know the headmaster is leaving for good and will no longer be with them at the school. Not a funeral, this ritual is more akin to sending an astronaut into space. All systems go! The headmaster gathers his inner circle, says his piece, leads the group into meditative states that dissolve the boundaries between people, submerges himself in the basin, opens the vessel, then, fully trusting his power and training, grabs for the next frame.