Let people know that proceeds of your t-shirt sales are raising funds for your dance companies. Post your sample shirt in a prominent location where students and parents can easily see it. Also take orders via newsletters, flyers, etc. Parents are more likely to purchase a shirt with their child's name printed on it as a keepsake. Set an order deadline to encourage closing sales.
Some studios charge a bit more for shirts ordered after the deadline.
Re-order forms will be enclosed with your shirt order.