Shock-rocker Alice Cooper has surprised the audience at "Rock of Ages" by joining the cast of the West End musical on stage.

The 64-year-old delighted spectators in London on Tuesday night by performing his classic hit "School's Out" to mark the anniversary of the track going to No. 1 in the U.K. charts 40 years ago this week.

Unfortunately for fans of the musical, Cooper's appearance is a one off.

"Rock of Ages," which was recently adapted for the big screen, is about an aspiring rock star and features cover versions by the cast of classic hits from bands such as Bon Jovi, Journey and Poison.