Stewart will play a private school headmaster in an episode titled "Learning Curve," scheduled to air in April.
Also appearing in the episode is Arrested Development alum Tony Hale and Hung's Jane Adams. Hale will play Rick Simms, a teacher who is fired from his job after being accused of inappropriate behavior with a student. Adams will play Simm's former supervisor.
Law & Order: SVU airs Wednesdays at 10/9c on NBC.