A valedictorian is a member of the graduating class (traditionally the student with the highest grade point average, although some schools have switched to other criteria, including direct voting by the members of the class) who is included in the rostrum of speakers as a voice for the graduates.
Since, legally, he or she is not an agent of the school, his or her speech is considered free and personal. In government-sponsored schools that do not schedule a benediction (rightly or wrongly concluding that to do so would illegally entangle the school in Church-State issues), many conservative Christian legal organizations have been encouraging valedictorians to include a benediction in their speeches as "student-led prayer."
Most professional chaplains know the difference between a short, inoffensive benediction and an over-the-top sermon disguised as a prayer. Many devout young people egged on by their families, their pastors, and the legal organizations, do not.