The story was something like this. The students attended an academy. They were like guardians that were able to tap into their past lives for power. The main character was kinda slow to gain power. His past life sister has a huge crush on him and a random sleepy girl also was going for him. They could use their power to change into armor and they pulled a necklace off themselves which then formed into their past lives weapon.
I watched this on YouTube the other night and now i wanna really follow it. The opening theme song video had them all fighting a dragon over a city.