After the end of his sophomore year, Jason was accepted into the Young Healers Camp, a summer-long program for prospective doctors. Grace Bowman was also accepted, but at the time, she lost her father in a plane crash, but Jason told her that she should go.
According to The Secret Diary of Ashley Juergens, Jason and Grace were placed into the same lecture group at camp and also did their hospital rounds together, causing them to become fast friends.
When they were picked to demonstrate the proper CPR techniques, Jason ended up giving Grace more mouth-to-mouth than anything and after that, everything they did was fraught with sexual tension until Grace had to give Jason a full physical and found his heart beating so fast that she thought he had high blood pressure.
They ended up kissing, but in her e-mails to Ashley Juergens, Grace said they ultimately decided to just stay friends. When they returned from camp, they were still quite close friends but not dating, even though Jason had broken up with Madison.
Although it's never directly stated, it's also indicated that Jason (like Jack) is on the Grant High Football team because during a conversation with Jack, Jason says he was at a party with Jack in which only the football team and the cheerleaders were invited to.