Slime-san to Yuusha Kenkyuubu

2000 years ago, there was a battle between a warrior and the demon king. Through this epic battle, the demon king won. Now, there are kids between the ages of 12 through 18 that are direct descendants of the demon king. They each have unique transformation powers that allow them to turn themselves into varying monsters, such as slimes and golems. Imaichi Butsuko and Hinase Suiren are both one of these monster kids. With the impending resurrection of the warrior, these kids enjoy their daily school lives while preparing for battle!

(Source: MangaHelpers)