is depression considered a disability under ADA LAWS ?

You need to have your son's doctor, who ever is treating him for the depression and anxiety, prepare a report that indicates that this condition is preventing your child from getting the education to which he is entitled.  It is not the condition itself that is disabling, but the effect that it has on your son's ability to attend school and benefit from the education.  He will most likely need a 504 plan or an IEP (Individual Educational Plan) that may include placement in another school.  But the first step is providing the school with some documentation that your son has this condition, and that a doctor is indicating it interferes with his education.