How do I address my juniors in college or school? If "senpai" means someone's senior, then if I am the senior what do I call my freshman i.e. the one who is younger than me in the same school or something. How do I address them?

To address younger kids in your school, you use their names and nicknames. You DO NOT address them as []{} unless it is jokingly done (and it is rarely done).

However, younger kids can address YOU as []{} either all by itself or by combining it with your family name like [].