I have had the same problem as you - ie I can cope with difficult classes and can maintain order. As soon as the agency know that you can cope with this they will persist in sending you to the same type of schools, because teachers who are willing to return are in high demand. I have usually found that the staff in "difficult" schools are great, and that teachers in "nice" schools are not very friendly, (obviously this is a generalisation).
My advice would be to think of yourself, it will take its toll having to deal with difficult children. Tell the agency that you will not go to really tough schools. You will quickly find out which schools you feel happy in. But, if you do persist with a tough school it usually gets easier. I had a class of Y5's once which the head described as the class from hell. I went to the school on a Wednesday and I was the third teacher that week. At the end of the morning the kids asked me if I would stay for the afternoon because usually supplys left at lunchtime! I stayed for the week and ended up teaching the class for 6 mths and then had a temporary contract.
You will have to be firm with the agency - good luck