Alien Nine Series by Hitoshi Tomizawa

In the future, alien invasion is a daily occurrence, and Elementary School #9 has assigned new sixth-grader Yuri and two other students to fight off the dangerous extraterrestrial threat. Even worse, she has to wear a creepy symbiotic alien helmet to get the job done - gross

It's the second semester and Yuri is handling the pressure of being in the Alien Party a little better now. Things are way more dangerous and the Aliens just keep dropping from the skies! Just why is the school sending sixth graders to fight vicious alien monsters?!

Yuri's friends are very different now from when they first joined the Alien Party...and they're not about to let Yuri see the things that they've seen. How can three sixth graders possibly stand up to a daily threat of alien invasion?

The girls have finally graduated 6th grade and are now in Junior High...but are they really prepared for the challenges that await them? There's a new girl in town and she seems to be hosting an entirely different kind of alien. Is she a new ally, or will she be Earth's latest threat?