Perhaps it's a new character who has a grudge against the dojo in general. They target Jack because he is one of the most prominent students. This new character could be someone who was rejected from the dojo in the past and is now seeking revenge.
There's also the possibility that it's an accident caused by one of Jack's own friends in the dojo. Maybe during a sparring session, someone misjudges a move and accidentally hurts Jack badly. This would then lead to a story of forgiveness and friendship as they all deal with the consequences of the accident.
Well, one possibility could be that during a karate tournament, Kim gets hurt while facing a tough opponent. Jack, being her friend and teammate, blames himself for not being able to protect her better. He then decides to train extra hard to avenge her loss and also to make sure she can recover safely. This could lead to a series of events where Jack discovers new techniques and also learns more about his own limits and strengths.
Maybe Jack gets hurt while trying to protect someone in the dojo. This could lead to a story where he has to deal with his injury while also facing the person or situation that caused the danger in the first place. His sensei could play an important role in guiding him through this difficult time.
He probably gets really protective. Jack might immediately rush to her side and check on her injuries. He'd be worried sick and would want to make sure she gets proper medical attention right away.
His friends from the dojo like Milton, Jerry, and Kim could be main characters. They would be the ones to support Jack after the accident and might be involved in any investigations or helping with his recovery.
Maybe Jack is seriously injured after being hit. His friends in the dojo might then rally around him, trying to cheer him up during his recovery. They could also start an investigation to find out who was responsible for the accident.
One common theme could be friendship. When Jack is hurt, his friends might come to his rescue or support him through the recovery. Another theme could be self - discovery. His injury could lead him to learn more about his own strength and weaknesses.
It could be a criminal that Shawn has put away in the past. Seeking revenge, they manage to find a way to hurt him. Maybe it's a mastermind criminal who has planned this from prison and has accomplices on the outside to carry out the attack on Shawn.
Perhaps an enemy from a rival group. They might see Sakura as a threat to their plans, so they poison her to get her out of the way. Another possibility is a character who has been secretly harboring a grudge against her for a long time, and finally decides to take action by poisoning her.
In this fanfiction scenario, if Kim gets hurt, perhaps her friends in the dojo would rally around her. They might take her to the hospital or a local healer in their 'Kickin It' world. Jack would likely be especially worried and try to find out who was responsible for her injury.
His coach might help him. His coach has the experience to deal with such situations.