At what point should an injured student see a sports trainer?

Before resuming play. All injuries in athletes should be evaluated by a trained health care professional prior to return to participation. As a rule of thumb, the sooner an athlete is diagnosed and treated, the sooner he/she can safely return to play. A certified athletic trainer is the qualified individual most often responsible for initial evaluation of the injured student athlete.
Sooner than later! Sports trainers are a significant first line of defense when it's comes to injury prevention, recognition and treatment. If you have access to a team trainer, definitely take reasonable advantage of that convenience.