I don't agree that April left Jett with a scar
—Jett left himself with the scar! As I have read it, April took a chance and bravely kissed Jett, letting him know without a doubt how he felt toward him. It was Jett that pushed April/Theo away, said bad things about him to other kids at the camp, and ignored him until April left.
April absolutely needs to come clean about the situation but he also has absolutely every reason to fear a negative outcome. They were just kids, but all April has to go on is the personal history and knowledge that Jett broke his heart once before already. 😕