I know what you are talking about.... I have a study group and limited time. At the beginning of this semester I noticed that my peers who normally study were not doing so! If
they showed up to our sessions (sometimes we made dates and they did not even bother to show) I noticed that we were covering BASIC material that he/she can cover on his/her own! Plus, like you I was tutoring rather then exchanging ideas!
Nevermind the fact that they did not even bother to read the material. So guess what? I got nothing out of the group sessions.
Fortunately I figured out their poor behavior weeks before our first two exams and decided not to attend study sessions right before our exams. I contacted my peers a week prior and let them know that I was not attending study session. In addition, I also hinted that I was disappointed that no one seems to be studying, but the reason I gave to not show up was unrelated to spare their feelings.
BTW, I do not mind leading study group, I mind being the only one prepared to discuss the material. If I am the only one, then I am talking to myself and a complete waste of my time! One study partner pointed out that I get to review of the basics in that situation. I quickly informed him that I can review on my own in 1 hour, which is much faster then I can do in our group!
In any case, I suggest that you tell them that you are concerned with the trend of them being under prepared. In addition, if they do not get their act together right before and exam, then do not show to up to study session. Be kind and make sure to let them know that you will not be there ahead of time. You DO NOT have to give them an excuse other then you are busy all week!
Do not get me wrong, my peers were upset at me initially because they either barely passed the first two exams or they did not pass at all. However, I pointed out some obvious flaws in their thinking. For example, I asked them if they expected me to learn the material for them? If they do not read, how were they going to benefit from me showing up to study group? Sure I can break stuff down, but if you did not read or bother to begin learning basic concepts how do you even know what I am talking about? After my discussion and their poor grades I am seeing improvement! My peers read ahead of time and we can now discuss the complexities of the material during our sessions. Not to mention that everyone shows up when we set a date and time. Hopefully you will have similar results. GL!