Kids don't race
Most high school and college age kids who run, run for their school teams and don't participate in road races when events conflict with their school racing schedules or when their coaches tell them not to.
In many races, there are no competitors in the 15-19 year old age group.
Running is usually an adult activity. In most races, the largest age groups are 30-34 and 35-39.
Still, some events market successfully to runners. The Allen Park Street Fair Run, in Allen Park MI, has created two new events specifically for high school runners.
Other races that attract kids usually negotiate with school track or cross country teams beforehand, so they know when they'll have younger participants.
Kids registering for road races is the exception, not the norm. It's best not to build your marketing plan around young runners unless you've pre-arranged their participation.