Mahoutsukai Tai! (Magic User's Club OVA)

Synopsis

One sunny day invaders came from space. They weren't little green men or funny girls wearing school uniforms, though. They were ugly robots which defeated the UN forces in seconds. After that the invaders (known as "eyeballs") roamed around and observed human society. In fact, they were peaceful unless attacked. They were polite and followed all the traffic laws, too. Still, they were aliens and the people of Earth wanted them to go home. The Kitanohashi High School Magic Club took it on themselves to defeat these foul robots and free the world. (Source: ANN)