Communication between students and teachers is usually extended beyond the classroom's time and space. This communication, referred to as out-of-class communication (OCC), may impact students' academic, social, and emotional development. Today, OCC is facilitated via social media and instant messaging services, which may affect its nature. This quantitative study (N = 155) aims to analyze the impact of WhatsApp™-based OCC between middle- and high-school teachers and their students on two variables that are key to learning and teaching: Teacher-student relationship and classroom environment. The data was collected through teachers' self-reported questionnaires. Overall, we find that despite manifested somewhat differently from traditional OCC, teachers' use of WhatsApp for OCC is also associated with better relationship with students and with better classroom environment.