The problems are many.
First, you never know exactly who will come out for auditions. Will you cater to just the upper grades, or will it be open to all students? Do I need to cap the cast to a certain number? What if I only have girls try out?
Second, do I have a decent budget that can handle buying a popular show, providing costumes, props, and sets? Do I have access to enough body mics (or any mics) and have a decent mixer and sound system for playback?
Third, will I have any helpers? Choreographer, music director, accompanist, sound tech, stage managers, costumers, set designers, etc.?
Fourth, how do I plan rehearsals? Will I have access to places to practice music and dancing? Will we have access to work on the stage?
Fifth, do I even know what kind of show would be good for my potential actors? Choosing a musical before you see the kids is an act of faith. But you need to choose one for marketing and advertising purposes well before auditions.
And the list of questions goes on and on...
Directing plays at a Church or Community Theater, as a director you go about the process usually aware of many of these situations. You pretty well can count on getting help from the community you are dealing with. You probably already know the stage and technical difficulties as well. You may already have access to costumes, props and set pieces you can adapt and use. You may be well acquainted with the sound system and lighting situations.
Directing shows at a school may be altogether different. You may be forced to hunt or beg for helpers from the other teachers and aides or from parents. None of them may have the talent, time, or inclination to be as dedicated to your project as you really need. (You may also be pleasantly surprised! I have.) You may need to be very clever with how you allocate time for rehearsals and lean on some High School kid volunteers.
My advice is to start advertising for helpers early in the process. Depending on the sizes of your show and cast, you will need at least 4 dedicated helpers --- assistant director, choreographer, music director/accompanist, and costumer. (If you can get stipends to pay them, that can help.) Other jobs like stage managers, set designers, dressers/make-up/hair, backstage monitors, sound and lights techs, etc. may be done with more part-time volunteers. Sometimes, parents or older siblings are wonderful volunteers, but they do like to be able to see the show.
If you are new to directing, good luck to you. It helps if you start with doing less complicated shows with smaller casts. It also helps to have your helpers in place so that you can have meetings with them to get on the same page early in the process. Having a show that can be rehearsed in smaller bits can be useful, too.
Here are a few suggestions for flexible, easy to produce shows from my catalogue:
"Stone Soup," "The Musicians of Bremen," "The Tale of Chicken Licken," and "The Frog Prince."