‘Mean Girls’ was a movie before Twitter even existed. How did creators update it to be a modern musical?

Who knew you could make a Tony-nominated musical from a self-help book for parents?

Tina Fey, that’s who.

“Mean Girls” picked up a leading 12 Tony Award nominations on Tuesday, including one for best musical and another for Fey’s book, which adapts her 2004 movie for the stage. But as “Mean Girls”…