A friend and I just got done doing a huge research project on Columbine.
A common misconception about Columbine is that Dylan Kebold and Eric Harris (the two killers) brought guns to their school and shot students and teachers because they were being bullied etc at school. That's not true. They did it because they waned to make history. They wanted to make it the biggest massacre in America's history.
They planted multiple propane bombs through out the school, mainly in the cafeteria. Their initial plan was to wait for these bombs to go off, causing the library from above to collapse onto the cafeteria. In this commotion they knew people would start running out the front doors, so they had planned to be standing there with their guns ready to shoot at the students running out of the school.
After the propane bombs they planted in the school failed to explode because they were badly built, they decided to just run int he school and shoot people mostly at random. They killed about two or three students outside of the school and then went inside and opened fire. Int he library they killed the most students, including themselves.
The reason this is still remembered today so very much is because it had a huge impact on society. Firstly, it had a very large impact on school security. Right after Columbine schools started implementing higher security, such as metal detectors, school IDs, video cameras, "no book bag rules", and even on campus police officers. Also, there were songs, books, movies, and other media based works created after Columbine. Columbine had a much larger effect on society than most people think it did. It was really the school massacre to change society, and that's why it's so remembered today.