Understand why you are going. Your contribution must bring a difference to lives. Usually, simply hanging out makes little difference. Be specific in what you want to accomplish. Understand the culture of the school.
Schools vary in personality and style.
Do your homework on the school system and the school leadership. Learn all you can from the newspaper, interviewing parents and students, even going to school system administrative offices where public documents can help inform you. Have a vision for the campus. Without passion for the campus, other ministry outlets will grab your heart. Have the personnel resources.