Rene,
There are so many questions raised by your post (questions that you are apparently asking as well) that I don't know how to respond. First -- I hope you aren't thinking that any student who barely passes or who fails a class are likely students with disabilities. Disability and poor academic performance are not equated. You say that "Many students claim to have a learning disability..." but it is unclear whether that "many" means many of the students you see or many of the students among the population of those who are failing. As to providing documentation to prove the disability -- that is something that should be provided to someone OTHER THAN THE INSTRUCTOR. If your school doesn't have someone to take responsibility for this, it should. To ask YOU to know how to read and interpret the information in that documentation is unfair, and to ask the student to provide documentation to each instructor is a significant problem LEGALLY. As to why students who believe they have disabilities may believe it entitles them to special treatment -- it depends on the age of the student. If these students are coming straight from the K-12 system, they may have been set up to expect just that - special treatment - by a K-12 system that has been providing it for years. Unfortunately, nothing is ever easy about all this.
Dr. Jane Jarrow