About a quarter of post-primary schools here are benefitting financially, or in kind, from directing students to buy specific PE wear from certain companies, Evening Extra has found.
Over four years, schools benefited from commission, donations, free kit for staff and mental health coaching.
The education minister said that she was seeking legal advice on whether she can introduce a price cap on the cost.
Michelle McIlveen said she was "really concerned" about "a very real issue".
