Fifty percent of children with ADHD also have learning disabilities. It sounds as if this may be the issue for your second-grader. Problems with reading and reading comprehension are suggestive of a learning disability, not of ADHD.
You should send a letter to the principal of the school requesting a meeting to discuss the reading difficulty. Under IDEA (a federal law), schools must consider evaluating for learning disabilities if there are problems such as the one you describe. If the principal refuses, find an education advocate to advise you or contact the Learning Disabilities Association of America.