None of us can answer this question accurately because each school has their own traditions and dress codes for promotion ceremonies.
As a middle school teacher I've seen promotion at a couple of schools. My first school was more formal for attire. There were rented, white, wedding type chairs, pretty flowers, and it was out in the courtyard. Boys wore suites and ties, or shirt and ties with no jacket. Girls wore dresses like you're describing but NO STRAPLESS. (really, if they came withstrapless and had no sweater to put on over, they had to put a loaner shirt on... ugly). My next school, boys wore khakis and dress shirts no tie, and girls wore sundresses and flat shoes. Again, NO STRAPLESS.
Ask at school. They will know the answer.
HTH