When your children are young, you were able to make all the decisions about them. Once you have teens, things get a bit more complex. Teens typically have their own schedules. They are learning how to become productive adults and have to be given the freedom to make some of their decisions on their own. This is complicated a bit when you have a child custody agreement in place.
One thing to remember is that the parenting schedule you had when the teen was younger probably won't work. In most cases, your teen will have a lot more activities as they get older. This might work against the child custody agreement terms so you will have to look into getting a child custody modification that meet the child's current needs.
If you and your ex live close to each other, it might be possible for your teen to manage their schedule no matter where they are on any given day. Both parents should be kept in the loop about your teen's activities and events. This can make it easier to plan schedules.
Some parents find it easier to let their teens devise a schedule for spending time with both parents. This only works when the adults can communicate, as they will need to verify the time is spent with the other parent or at special events.
It might be beneficial for you and your ex to discuss some basic ground rules that will apply to both homes. Some of these might include curfews, academic expectations, dating and driving privileges. Once joint rules are established, both parents can enforce them equally.