To even consider anything like this, you need to be a licensed therapeutic riding instructor. Also, the liabilities involved in doing this would require specific types of insurance in case anyone would be hurt. Also, you are too young to be considering this. A lot of parents of children with special needs would be very leary about putting their child into the hands of another child. Also remember, not every potential rider will be a child. Believe me, I am a special education teacher. I work with severe/profound students.
I would suggest, if you are really interested in getting involved with therapeutic riding to contact a center in your area and possibly volunteer there. It would be a great experience and you would learn a lot of things. This will be your best bet and in this case everyone can and will be safe.
I do not know what part of California you are in, but I have attached a link to all of the centers
in CA that do therapeutic riding. I would suggest if at all possible to volunteer at a NARHA Premier Accredited Center.
I do not mean to sound harsh or try to upset you. But I want you to gain a good experience and to keep everything safe for anyone involved, including the riders with special needs, the horses, and yourself.