How to keep your students in class - tips to teachers

As a math teacher, I had 50 minutes to give the main subject, to present it in an interesting way, to keep the attention and, the more important, to keep the students inside the class.
I used this method with children from 10 to 13 yo and it resulted incredibly successful.
I had a class money. A piece of paper, signed by me, that valued one way-out. The student, by instance, needed to drink water. He or her would give me one of the papers, and I wouldn't even look angry. The only rule was that I shouldn't be in an important moment, as to presenting the mainly subject of the day. All of the students received four papers each month. As I had five classes each week, they would have the possibility of going out four times in a total of at least 20 classes.
During all the time that I used this method, I haven't got even one student who used the four papers. The reason was that each group of four papers valued 0,25 points in whatever test grade the student chose. If the student had received the grade 8, it would make it 8,25.
Other teachers didn't understand it. I even once received severe critic, but I can tell my students soon created ways to not going out in our classes. They would bring bottles with water, as I used to encourage them, for example.
There were a lot of homework to make the papers in a unique way that no one could imitate. I printed the name of the month, and signed each one. Afterwards, I covered with plastic paper.