I think that a prank can cross the line into bullying before it even happens. If you are pranking someone with a prank that you are sure won't offend or harm them, it's just a prank. If you are trying to harm them or make them feel bad, then that's bullying. Sometimes people accidentally take things to dark but it is forgivable if they started with food intentions and did not mean to do so. Sometimes people overreact about pranks, but as long as you weren't trying to upset them , I think it's fine.
I think pranks are pretty funny, but sometimes they do get out of hand. We used to prank each other in the locker room every day after gym class. It got so out of hand at one point, everybody had to hide there belongings, just to make sure nobody messed with it. Every day was the same thing, but it was all fun and games. Sometimes it got a little out of control, but nothing more than a little bit of wrestling. One day it did go too far though, and the "joke" was on me. I walked in on the other kids hiding my stuff, and as I tried to get some of it back they tossed it back and fourth around the room. Then one kid tried to pull my backpack out of the locker and ripped the whole thing wide open. A brand new expensive backpack. As I ran at him, he then knocked over my laptop that he had already taken out of my backpack and broke it. I wouldn't consider that bullying, but that was a prank that got way out of hand and ended with a cost.
Another locker room prank that got out of hand was when one student put a condom that looked like it had been used on top of another students backpack. The two students got in a heated debated after that, because they already had beef, but the prank broke out into a fistfight that eventually had to be broken up so nobody got in trouble.