Edit one of the Other types to be School Zone. If you want to be notified of zone begin and end, you would want to edit two of the other types.
When driving into the school zone, press the mark button (red gps marker icon with + inside). If you have sabre plug-in installed, press the "user marks" button next. Then press the arrow up button in the school zone row so you get alert only in that direction. You should do the same thing when driving into the school zone from opposite direction.
If you want school zone end alerts, you would need to repeat above steps when leaving the school zone, except choose the zone end type.
If you want same alert for zone entry and exit, you could just mark the entry and exit with same alert type, but select the alert type button for alert regardless of direction.