This article is from the Theatre FAQ, by aku@leland.Stanford.EDU (Andrew Chia-Tso Ku) with numerous contributions by others.
Some Broadway shows sell student tickets; often these shows have run
awhile and use these offers to attract business. [Notable exceptions
to the rule were Cameron Mackintosh shows, which sold student tickets
(albeit in the rear balcony) to shows like LES MISERABLES and PHANTOM
from the very first performance. Since revamping the ticket scales to
$15-$65 for everyone, student discounts for the CM shows have been
eliminated, but student discounts may still be available for his
touring productions.] One often has to inquire about these tickets at
the boxoffice, as they are not posted to the "Scale of Prices" sign
outside the boxoffice. Student tickets are only sold in person at the
boxoffice, and require a student ID. They usually aren't prime
Student tickets are more readily available off-Broadway, especially
at the non-profits like the Public Theater. Some offer only
student/senior rush seats (ie. 30 minutes before showtime).