Public school districts are required by federal law to offer reasonable accommodations for kids with ADHD under section 504 of the Federal Rehabilitation Act of 1973, a civil rights act. Your child's teacher can put you in touch with the appropriate special education administrators in your school district.
Children may also qualify for school services under the Individuals with Disabilities Education Act. Your school district's special education team will know which law applies to your child. Children who qualify for services under IDEA (IDEA can sometimes provide more extensive accommodations than a 504 plan) will receive an Individualized Education Plan or IEP that will spell out just how the school will accommodate your child and what their goals for his education are. The plan will be reviewed and needed changes made each year. For more information, go to http://idea.ed.gov. Even preschoolers can sometimes qualify if they are identified through Child Find, the government early intervention program, although ADHD is most often diagnosed in school-age children.
Parents may be hesitant to have a 504 plan or IEP for their child for fear of stigmatizing them. (This is probably a leftover from the days when "special ed" was segregated from the rest of the school and often a dumping ground for anyone teachers couldn't deal with.) But a 504 or IEP is really just a way to make sure the school is working together as a team to help your child achieve maximum success in school. It also offers parents legal protection and remedies if the school does not make needed accommodations. Teachers and administrators are not going to share this information with students. And remember, ADHD is such a common disorder that it's likely numerous children at your child's school will have the most common accommodations, like extra time on tests.
Parents who choose religious or other private schools will have to organize accommodations outside this official process (unless they are referred to private schools because the local school system cannot accommodate their child). These schools may not offer the same level of support for ADHD children. However, they may have smaller class sizes and other advantages for children with ADHD. Parents will have to evaluate the differences and the cost, and make the decision.
You can also encourage your child's teacher or principal to attend a seminar or workshop on ADHD, where they can learn more about behavior modification and other strategies to help their classroom run more smoothly for all students.