I guess it depends on what perspective you have. If this is from the perspective of the school, a reinforcement of extracurricular activities would be the addition or growth of more extra curricular activities. This would mean to have a greater variety and quantity of the activities.
If this is from a perspective of a student, then to reinforce your extracurricular activities would be to perform better in each one. It could also mean to join/be more involved in other activities.
