According to Marczyk et al. (2005), the independent variable is the factor that is manipulated or controlled by the researcher, while the dependent variable is a measure of the effect (if any) of the independent variable. Typically scholars are interested in exploring the effects of the independent variable. In its simplest form, the independent variable has two levels: present or absent (Marczyk et al., 2005). In the question: Do mobile devices negatively impact classroom attentiveness of high school students? The independent variable is mobile devices; the dependent variable is high school student concentration levels.
Marczyk, G., DeMatteo, D., & Festinger, D. (2005). Essentials of research design and methodology. Hoboken, NJ: John Wiley & Sons.