Their training time is not restricted by time constraints of school hours, study time, commuting, other commitments.
They are not dealing with the stresses of school and school related responsibilities, time commitments, etc.
They have more down time for sleep, recovery, massage, supplemental exercises, relaxation, etc.
They can more optimally pick and choose when they want to run time trials based on factors such as training progress, weather reports, available pacers, etc.
They have more opportunities to plan for optimal TT race efforts due to not having to adhere to a structured racing schedule.
They can run more high quality time trials due to absence of tactical races, crowded fields, etc. (due to absence of races).