Hi JetaSenpai,
You can visit the Combat Ground building and press the war button to change your defence set up. You CAN use the same defenders to plunder others, but if you use them to occupy somebody after plundering them, they will not be available to defend your school.
On a separate note, I would like to extend my invitation to all the Call of Arena players here to join our alliance - Ahchoo. We're pretty new, our alliance was formed by four real life friends and we're now between level 32-39. Our alliance is at level 2, and we would like to advance further. We need active players that would like to join our circle of friends and help this alliance unlock more functions and continue to level up. Considering that we only had 4 people for the last alliance war and we ranked 3rd in our arena, I think we're doing okay. Please send a message to Breadtruck in game or request to join Ahchoo if you're interested.
Thank you!