Research the musical and spend time planning how you want to stage the production. In researching the musical see how other directors and high schools have staged it in the past and, if a period piece, understand what was going on in the world at the time the musical is supposed to take place.
Create a rehearsal schedule that will run six to eight weeks. Rehearsal schedules need to include days off, fittings and tech rehearsals and take into account the other activities and homework load of the high school students in the production.
Distribute scripts, musical numbers and schedules to cast a week before rehearsals are to begin.
Use the first three weeks of rehearsal blocking the script and musical numbers.
Give the cast notes after each rehearsal. The notes should cover things they need to work on for the next night's rehearsal. High school students can sometimes be procrastinators. Be firm about deadlines and any repercussions that may take place if they don't follow through with the notes and directions they are being given.
Take actors off book after three weeks. If actors need prompting, the stage manger can prompt them as needed but even high school actors need to know their lines by this point.
Spend dress rehearsal making costume adjustments, fully integrating props into the production and making any changes in blocking or to the set changes that may be needed. Dress rehearsal is about getting the timing of your production just right and it's your time to fix the musical before it goes in front of an audience.
Deliver an opening night speech to your high school cast an hour before show time. The students need to hear from their director that you are proud of the work they have done. They also need reassurance that they are ready to put on the musical and that they will be great in their performances.