To have a successful teaching career, you must remember this; your job is to take every student as they are and teach them as much as you can. Changing what goes on before you get the students is a very difficult task, as is changing what happens after the students leave you. It is very frustrating when you realize that you are being judged on student achievement that in many cases does not reflect your teaching.
Work with the curriculum committees and textbook committees to try to bring about long range changes, but don't expect anything to change quickly. Try to get students scheduled into the appropriate classes, but ultimately you have to play the hand that you are dealt.