The American Psychological Association explains in this article many of the benefits of SEL programs for students. My favorite is that the benefits of SEL training continues long after the students are finished with the program.
RAK is a full-service website with a FREE SEL program you can use with your students. There are lesson plans, resources, videos, and everything else you might need to begin a program in your classes. The Educator Guide is excellent. Did I mention that the program is completely free?