"Adversely affecting educational performance" is a phrase from the IDEA (Individuals with Disabilities Education Act). The interpretation of this phrase has been debated for many years. Many students are doing fine academically, but still have a speech impairment. Because school systems often include adequate oral communication as part of their performance standards, children with only a speech impairment can receive services.
You may find helpful information from the American Speech–Language–Hearing Association (ASHA).