Tennis: backswing loop

Helo, my name is Fernando, I am a Spanish tennis teacher who started teaching six months ago. I am working in a public sportcenter and I am having some problems with adults. When I started I realiced that the students were used to playing tennis without any control. Young students told me that the other teacher only used to leave them play - he rarely explained them how to hit the ball. Consequently the do enjoy my clases and they are learning a lot and impoving their level of tennis. So far so good, but the problem is that with the adults I have the impression that they don´t like my clases. In fact some of them left the group at the begining. I try to mix different kinds of drills during the class and I usually finish it with a game like for example 21, winner........ But it seems that they don´t enjoy it. During the exercises most of them don´t try to do what I tell them. As a result, they don´t have a good level of tennis. But as far as I know, the other teacher was a tennis player who uses to play against them during the clases. As this is my first time teaching tennis, I don´t fell confidence and that is the reason whay I do´t like to play against them. Appart from that I think that this is not my role as a teacher. So I would like you to tell me how to deal with that situation. I don´t care if they leave me because this is not going to affect my job, but i wouldn´t like them to leave just because I like tennis and I like teaching tennis. Thank you very much for your help. I am sorry for my English. Best wishes!!!!