“Lemonade Mouth” author Mark Peter Hughes” signed copies of his book “I Am the Wallpaper” at Girls Inc. in Lynn on Thursday. The event was part of the United Way’s summer learning program, which aims to keep kids reading during the summer. “Lemonade Mouth” turned into a Disney Channel original movie starring Bridgit Mendler, also of “Wizards of Wizardly Place.” “Wallpaper” was Hughes’s first young adult book.