Ключевые слова: back to school, background, billboard, black board, blackboard, board, chalk, chalk board, chalkboard, class, classroom, clean, college, communication, earth, eco, eco friendly, ecofriendly, ecology, education, elementary, environment, friendly, global, go green, high school, highschool, info, lesson, message, natural, note, notice, notification, organic, school, university, warming, write,