-san is your safe bet
-chan is cutesy, and familiar
-kun is more endearing, and familiar. A teacher becomes familiar and endeared with their students throughout years of working with them, which is why it is used there.
-sama is reverent. "My lady"
-dono is also reverent. More so?

Regan's totally comfortable with her sexuality. As if groping Kotone didn't show that enough. =P

Fabriiii, what is this important thing you have in stooooooore? *flops*

Incidentally, had you wanted Regan to go to the other teacher? >>;