Doufu Mayoi 
Black Summoner: Volume 16 

Peacetime continues, and Kelvin is just about to go stir crazy when a certain problem falls into his lap—a family problem. Rion wants to go to school, which would drive Kelvin and Gerard absolutely mad with worry and loneliness. Not only that, but in the course of things, she manages to involve Dark Mel and Bell in the plot as well! What awaits the girls in the Academy of Lumiest? And how will Kelvin and Gerard fare without them?