Though I am a speech-language pathologist, when I was an undergraduate student I enrolled in special education classes to earn endorsements in various areas of special education. I earned my mentally retarded (now cognitive disorder) endorsement which also allowed me to teach pregnant students, my emotionally disturbed and learning disorder at undergraduate. I started my master program in generic special ed which allowed you all areas of special education, but I think they did away with that, only to change my major to educational diagnostics. Not sure if they still have diagnosticians as most want Licensed Specialists in School Psychology.
Anyway, to work with special needs children in public schools you must first receive your elementary or secondary teaching certificate. You then work on earning endorsements at the same time as your undergraduate certificate. Your last semester you will student teach for 8 weeks in regular education and 8 weeks in special education. After graduation you will take teacher certification testing by the state. I believe you will have the general education exam and then you will take exams in all the other areas you wish to acquire. I believe this might be true, but I cannot be sure but you can challenge a test. This means that you may not have course work in the field but have knowledge and you may pass the test to become certified. This is only for teaching credentials.
Good luck. Though I am a speech pathologist, my love was working with autism, cognitive disorders and multi-handicapped students. You really have to have a special heart to work with this population.