Boston-based artist, former Newbury Street gallery director, and founder of the Plein-Air Art Academy Diana Stelin is preparing to teach kids pop art in the spring. The Academy is also running a first-of-its-kind Pre-K/Kindergarten art contest with entries due March 31. In Mid-April, you can join the schools’ immensely popular plein-air painting event: Cherry Blossoms at Arnold Arboretum. And during ArtWeek Boston,…