Check the district regulations. Start by finding out what guidelines, if any, your school district has for high school productions.
Consider your talent. Make a list of how many lead-quality students you have available. Note their voice ranges and their general appearance. You'll need this information when you start considering titles. It will help you decide whether you're looking for a musical with a large ensemble cast or a smaller group or even whether you have the ability to put on a children's show.
Research musicals. Attend performances of musical theatre whenever you have a chance. Regardless of whether the local talent company is putting on the show or it's a professional, national company, attending performances will give you some good ideas. You can see different staging and view the plot and characters for appropriateness to high-school students.
Leak a few titles. Which musical your high school performs may be your choice, but the popularity of that decision could make or break the success of the show. To get a good read on the population, start a rumor about a couple of titles you're considering. Sit back and listen to the feedback, then make your decision.