A study involving 629 12th-grade students from three Los Angeles-area high schools has revealed that, across both genders and all ethnicities, adolescents with more in-school friends, compared with out-of-school friends, had higher grade point averages. It’s assumed that this is due to the fact that in-school friends are more likely to support school-related activities, including studying.
Reference
Witkow, M. R., & Fuligni, A. J. (2010). In-School Versus Out-of-School Friendships and Academic Achievement Among an Ethnically Diverse Sample of Adolescents. Journal of Research on Adolescence, 9999(9999). Retrieved from http://dx.doi.org/10.1111/j.1532-7795.2010.00653.x