GTO stands for "Great Teacher Onizuka," after the central character, Eikichi Onizuka, who would like to be the "greatest high school teacher in the world."
Back to the basic plot: Eikichi Onizuka is a new teacher, without full status yet at his school, so he must prove himself constantly. At one point, to save his job he must take a standardized test with his students and get the highest score in the country. Being new and so young, he has unusual ways of reaching his students, and this sometimes causes resentment from a few of the students.
GTO is a fun way for a non-otaku to ease their way into manga and anime: the teacher-student scenarios are easier to relate to than stories with characters who wake up one day with magical powers.
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I love GTO! It's probably one of my favourite anime's, and I'd love to read the manga since the anime version ended craply. xD