Aim/Purpose: To study associations between elementary-, middle- and high-school students' perceptions of classroom environment and student-teacher relationship and their out-of-class communication practices via WhatsApp app. Background: 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 have impact on its nature. Methodology: Methodology was quantitative in nature. Data was collected using an online questionnaire (implemented on Google Forms, http://forms.google.com, during June-July 2016. Participants (n=300), from 5th-12th grades (11-18 years old), were recruited in schools in a few Arab villages in northern Israel, with the assistance of their teachers. Contribution: The present study expands the growing body of knowledge about student-teacher communication via online social networks, specifically regarding out-of-class communication. We identify the unique aspects of WhatsApp-based out-of-class communication, which shed light on student-teacher relationship at large. Findings from this study may assist educators (while in training and/or professional development programs) to reflect upon their own educational agenda and to check if and how they and their students can benefit from OCC. Findings: Overall, we identify WhatsApp's important, unique role in promoting good student-teacher relationship and positive classroom environment. Recommendations for Practitioners: The findings regarding the unique contribution of WhatsApp to student-teacher out-of-class communication should be taken into consideration by policy makers while formulating policies for the use of online social networks in educational settings. Teachers should be aware of the important role this type of communication plays for their students and for their classroom. Both teachers and students should communicate respectfully, with teachers serving as role models for their students regarding proper digital behavior. Recommendations for Researchers: This study should be replicated to more populations and to more communication platforms, in order to validate its findings. Impact on Society: The associations between out-of-class communication via online social networks and student-teacher relationship have two main effects on society at large. First, promoting better student-teacher communication could improve learning and teaching. Second, if this communication is to be carried out properly, the students -- who are the future citizens -- will learn how to behave correctly in the digital age. Future Research: It is advised to explore the studied associations in other populations and regarding other communication platforms. Also, qualitative exploration is advisable, as it may shed more light on the unique aspects of WhatsApp-based student-teacher out-of-class communication.