sometimes some students bully other studensts that are weaker than them. It makes me very angry. = It makes me very angry whenever I see students bully other students who they think are weaker than them.  
it's unfair and mean action. = It's unfair and it's mean. and Bullying in school is considered criminal in Korea. = Also, bullying in school is considered a crime in Korea. They don't notice that they might become a victim like someone whom they was bullying. = They may not notice but they could also become a victim of bullying.

Note: Always begin your sentence with a capital letter.