As far as I know, nobody can guarantee IELTS results.
According to my personal experience most of the IELTS classes are scams and target on students moving for academic purposes. They don't target higher bands wanted to apply for PR. But at the time you register they don't tell you about that.
Most of the time academic requirements are lesser, lesser than 6.5, and sometimes accept average band not a particular band for each format. So the student who has some knowledge in English go over it. And the other thing is we don't know the pass and fail rate of a class.
Most of the tutors trying to do Reading and Listening lessons in the classes (we can do this by ourselves) and seldom do Writing and Speaking which are the most important and difficult to pass.
Nobody try to cover up the basics like grammar. I have met some people tried IELTS more than 10 times.