Kanzen Shouri Daiteiou

When an alien force attacks Earth, a giant sword falls onto the campus of Paradise Elementary School, transforming it into a flying vessel. The fate of the planet lies with one group of kids who must pilot powerful mecha to stop the invaders before it's too late!

(Source: Anime-Planet)

Note: Originally planned as a TV anime series, this ended up as a promotional video for its manga serialization on magazine Monthly Daiteioh.