Visit the school. Depending on the nature of your disability, you should consider things like whether the school is accessible or whether it's close to accessible transit. Visit during a time when the school is the busiest. If you require the use of a wheelchair or other mobility aids to get around, this will give you a good idea of any challenges you might face as a student.
Speak with the school director. Some schools offer special classes tailored to the needs of students with disabilities. Group classes and group music therapy sessions are sometimes also available. The school director or a member of the staff could recommend a class or help develop a program catering to your needs.
Go in for a trial class. Most schools will offer one or more free classes to help you determine whether the program, instrument or instructor are right for you. Be sure to ask your instructor whether he has any experience teaching students with disabilities.
Get a private music teacher. Some schools will give you the option of having a music teacher from the school come to you. This may ease any initial inhibitions you might have about going to music school with a disability. If the school does not offer that option, they may still be able to recommend a music teacher based on your instrument of choice and your disability.