I'm having a hard time in school. My mom has agreed to let me live with my grandparents so that I can transfer schools. In Alabama does my mom have to sign over custody to do this? If so, how? If not, what do I do?

Welcome to AMHD. The answer to the custody question may depend on how far away from your mother you will be. There could be health insurance implications if you are covered under her major medical policy. Find out what documentation is needed, if any, in the school into which you are transferring.
About.com: http://pubs.caes.uga.edu/caespubs/pubs/pubsubj.html#Family (http://childparenting.about.com/gi/dynamic/offsite.htm?zi=1/XJ&sdn=childparenting&cdn=parenting&tm=20&gps=290_552_1276_865&f=00&su=p284.9.336.ip_&tt=2&bt=0&bts=1&zu=http%3A//pubs.caes.uga.edu/caespubs/pubs/pubsubj.html%23Family)
Scrowl down to "grandparents raising grandchildren series"