The legal issues have already been covered from several aspects.
My suggestions are this,
Find out what is behind your son's wanting to go to another school, so that whether or not he stays there, those problems can be addressed.
Many teenage boys have difficulities about this time for a number of reasons whether from divorced families or not.
During this time there will be changes in all your relationships because the child is maturing and testing their wings, yet still need the support of their parents in different ways than before, and for divorced parents, co-parenting will experience some challenges, this is when you both have to make an effort to work together.
Is your ex agreeable to taking primary custody?
Is child support an issue?
How far apart are your homes, is there a possibility to enrole your son in a school in his father's district and transport your child, without changing custody, sharing custody and/or trial the change?
Talk about this with your ex, discuss the school problems and try to reach an agreement between yourselves first and then with your son. Don't be surprised that if a part of the problem is discipline, rules and expectations that even if custody is changed that your son may not be so happy with the change if the same rules of conduct are consistantly enforced in both households. Consistancy and agreement between the parents is essential and alternate plans are a good idea.
Taking a well considered agreement to the judge will greatly reduce the stress and cost as opposed to a custody fight.
As a parent, he will always be your baby and your relationship will enudre, but as most have said, your child needs both parents, sometimes one more than the other at different times, this usually prompts accommodations in custody in divorced families.
Ah the joys of parenthood
