Well, everyone already answered, but I'll take the shot anyway. (Haha!)
Jimmy Brooks was shot late in season 3, and he's been in a wheelchair ever since season 4.
The character who shot him is now deceased, Rick Murray. Rick had a lot of psychological, and emotional problems. He was bullied a lot in his last few episodes for the girl he put into a coma. (Terii McGreggor, who was one of the ”cool” kids, back in the day.) [Rick's] final breaking point was this trivia show, “Whack Your Brain” held by the schools. Jimmy just so happened to end up on the cast of this game, as no one else had the ultimate knowledge of everything sports related. He and Rick began to bond a little, much to Spinner and Jay's dismay. (He had grown close to Toby and Emma as well. He developed a crush on Emma, actually. )
Well, the “rebel” clique decided on a prank – which was to pour yellow paint and chicken feathers onto Rick when he stood in the Winner's Spotlight. (Carrie, anyone?) After the cruel prank had been done, it was too much for Rick to handle any longer, as he thought he was finally able to obtain redemption from his peers for what he had done to Terri. He dashed into the bathroom to hide and clean off the paint. He quickly hid in the nearest stall as he heard Spinner Mason and Jay Hogart entering the bathroom-- laughing and discussing how funny the incident was. Well, before Spinner spilled too much info, Jay noticed that Rick's paint stained shoes were under one of the stalls and they proceeded to falsely accuse Jimmy as the mastermind behind the plot -- when in reality -- Jimmy wasn't even aware of it – at all.
So, it was a two part episode, as most of the “heavy” episodes are. After some obvious fuming, the last scene of the first episode was of Rick in his Father's study, looking at a gun in a decorative hiding case. The next episode showed his arrival to Degrassi later on that same day, still in his paint stained and feathered tuxedo, clutching onto his backpack. He wandered aimlessly through the halls, which many students noticed and commented on how odd it was that he didn't leave for the rest of the day. Rick finally ran into Jimmy at his locker. Jimmy considered Rick a friend at this point, sorta. Then Rick pulled the gun from his bag, babbling about being betrayed, blah blah.. Jimmy had a look of horror on his face, and attempted to run.. Rick aimed and shot Jimmy in the back. Not fatally, but still.
Rick then went back to the wandering; this time he found Emma! He tried to confess his feelings for her, and pulled the gun on her as well when she didn't share them. Sean Cameron dove towards Rick, and they struggled a bit until there was a loud BANG. Both fell to the ground, if memory serves right. Sean had somehow managed to turn the gun back on Rick, and either Rick pulled the trigger or Sean did. I don't think it was ever mentioned who actually pulled the trigger.
It wasn't as heavy as everyone portrayed it to be, and Jimmy SHOULD have died in the incident, it seemed like more of an Episode for Sean's character instead of Jimmy. If J.T. Yorke could die from a tiny stab wound.. Jimmy should have died from a bullet wound to his spine.
Jimmy went through the whole recovery process, physical therapy, and coming to terms with never walking again, before returning to Degrassi.
As a side note, being a critic, I felt this was just a recycled and modernized version of one of Caitlyn's (That blonde lady who's friends with Emma's Mother, Snake, and Craig's Step-Father.) boyfriends who committed suicide in the original Degrassi Junior High, honestly. He even looked like Caitlyn's ex. Kinda. They both had the long hair, roses and tuxedo. I say this because I think it would have taught a lot more about school shootings if more than the gunman himself had perished.
I hope that helped!