Please stay on task

Today was an especially frustrating day. The students that need more attention and assistance have difficulty working in cooperative groups. I've varied the group projects, but these students still have difficulty staying on task. Dr. Tharpe discusses mature classes, students that need less assistance and direction from the teacher; however, this class has very few mature highly skilled students to help direct the cooperative groups. How can I create appropriate academic group activities, move or "stretch students to the next instructional level, and keep all students on task?