Firework Crashes My Server

I have my own server and we have some people on it. Lots of people at my school play on it frequently. So i hosted a party. TONS of people came until the fireworks shot up. Then one by one you see () Left the game () left the game then all of a sudden you left the game. Then if you try to log back on it doesn't work. SO i tried the next day it was fine. I didn't know what made it crash so i reset the server and tested out some stuff. I made a firework. Simply placed it. RIGHT when i place it it just crashes. NOW i cant even get on my server and i am getting LOADS of notes and stuff from school saying your server doesn't work now.
PS There were NO mods/skins. Just plain MINECRAFT SURVIVAL like we like it.