Setting the time
The more often the study groups are held, the better they work. During the first time there will be more need for the teacher’s support, but later on, the teacher’s role will be more in the background.
When choosing the time, you should take into account the space you will need and other limitations. What is the best size for the group, so you won’t need to crowd control? It might be easier for students to teach peers with a friend. Or could the teachers ask the students help to figure out what kind of groups would work best?