Matchless Raijin-Oh(1991)

Zettai Muteki Raijin Ou

Suddenly, an evil empire, Jaaku, raids the Earth. But the light appears from the Earth, and fights against Jaaku. It is Raijin Oh operated by Eldran. Eldran has protected the earth since the ancient ages. But he was injured during the fight, and Raijin Oh falls to the ground. On the other hand, Jaaku scatters their weapons, Aku Darmer, all over the world.
Meanwhile, in Yosho elementary school, they have extra lessons in the 3rd class of the 5th grade. Eldran leaves Raijin Medal to them and asks them to save the Earth for him, and then he disappeared. In this way, the Earth Defense Agency consisted of elementary students is founded.

(Source: AnimeNfo)