Dialogue implies that the Spring Prom is held on a rainy night in April, but the newspaper that Larry reads the next morning is dated in mid-May.
Larry reads a newspaper delivered on a Saturday morning with a 1947 date that was a Thursday in that year.
After Ricky walks Cynthia home from the audition, the shadow of a moving boom mic is visible on the staircase banister as Cynthia discusses the play with Louise.
The call letters of the radio station that broadcasts the operetta from the fictional small town in Illinois were, in 1947, really the call letters of a radio station in New York City. It's highly unlikely that an Eastern metropolis would broadcast a high school musical from a Midwestern town.