This dream may symbolize a desire for intimacy and connection with someone from your past, represented by the boy from school. The fact that you were alone in the backseat and fell asleep on his shoulder suggests a sense of comfort and trust in this person. The kiss on the cheek and the kiss mark may represent a longing for affection and validation from this person. It could also symbolize a desire for a deeper emotional connection with them. The fact that your girlfriend already knew about the encounter and was shocked may suggest that you have been keeping this desire hidden or suppressed. This dream may be urging you to explore your feelings and desires for this person and to communicate them openly. It could also be a reflection of your desire for more excitement and spontaneity in your current relationship. Overall, this dream may be highlighting your need for emotional and physical intimacy in your relationships.