How do I keep a small-group discussion focused?

I am part of a high school small-group discussion at church. When neither the adult normally leading it nor his substitute could make it, one of the students started leading. The discussion quickly became distracted; one student was constantly doing something on his phone, and even the leader was on the phone with someone. He later let me take over, but the discussion was now so off-topic that I couldn’t get it back on topic. I did not have the attention of the entire group at any one time. What should I do the next time that happens?