Otome Senshi Lovely 5!

From Tsundere Translations:

Ogdoas Private Academy...
Under the new, young headmistress, freedom, independence and self-fulfillment are the motto of this mission school. However, among the nuns there those who are concerned about order and morals within the school and who oppose the head-mistress's ideas.
And without anyone knowing, right now, at this school... a battle between good and evil is about to take place!

A continuation to Shion and Nashida-sensei's story called "Teacher's Pet" can be found in Yuri Hime Wildrose volume 2