Ariel (1989)

To save the Earth from alien invaders and their giant monsters, Dr. Kishida does what any other red-blooded mad scientist would do: he builds a giant robot—the ultimate feminine fighting robot! Unfortunately, his granddaughters refuse to pilot it! Apparently, they've got more important things to do than becoming teenage super-heroes. Besides, how are you supposed to study for your college entrance exams while getting beaten up in battle?

The fate of the world rests upon two people: Aya Kishida, one of the doctor's granddaughters, who may have to give up prep school to pilot the mighty (and lovely) ARIEL; and a mysterious alien named Saber Starblast, who may have the power to defeat the invaders once and for all...