1. Allison Holker from So You Think You Can Dance was one of the back-up dancers
  2. Kenny Ortega, the director, also directed Newsies
  3. At one point, Kenny Ortega was attached to direct the movie adaptation of Lin Manuel Miranda's Broadway musical In The Heights
  4. Zefron didn't sing at all in the first movie. Apparently the Troy songs weren't in his vocal range, so Drew Seeley (who went on to perform in the HSM concert tours) sang his parts. Zac sang for both the sequels, though.
  5. The iconic "roller coaster" dance move in What Time is It from HSM 2 was concieved because of a "back massage train" the actors and dancers were doing in rehearsals.
  6. Kenny Ortega choreographed Dirty Dancing
    Suggested by   @crossedties