I agree with the above posters. Do not change schools his senior year to improve grades for college. It may have serious consequences not only in terms of admissions as other posters have mentioned above, but also in your son’s academic and social life (like backsliding because he’s having to re-adjust to a new environment or falling in with a crowd who parties rather than studies). If his GPA is upward trending at a respected high school & and with a rigorous schedule, then that is the basis for a good essay on how he put himself on the right track.