Wow. This is a tough situation. We go through something similar every year as teachers have to get to know my son (why they can't talk to the previous teacher I don't know), but we have had good luck in working with teachers to differentiate the instruction after the "get to know you period." I am a former principal, and I have seldom seen a long-term sub that could adequately provide differentiated instruction.
You might consider setting up an appointment to meet with the principal (or in some schools the guidance counselor). Be prepared to express your concern that your child is not being served properly. Even if the school does not have a gifted program, ALL students deserve instruction at their level. Bring examples of books your child can read (you might even audio tape him/her reading). You could request that your child be placed in another class for all or part of the day.
This would also help to build a case for grade skip later in the year if needed.
