I was checked for mental retardation and all the tests came back negative but I still am in special ed. Why?

Good question. Any learning disability, behavioral problem or other special needs might qualify someone for special ed even if their IQ is normal.
Check your IEP. There are 12 categories in which children and adolescents, actually through age 21, are eligible for special educational supports. Your eligibity category is listed on your individual educational plan that was created after you were tested by your school district. Ask your teacher or school counselor to explain your iep to you. Teenager's parents can request that they participate in their own iep.