Dress comfortably but dress neatly. Be sure the principal knows your name and when you are to arrive. The principal may be able to assist you with getting what you need for your presentation more easily than a teacher. When you arrive, go to the office, introduce yourself. Explain why you are there and who you are there to visit.
Many schools have strict security arrangements.
Your contact at the school should make you aware and help you through them. It is advisable, if possible, to spend some time beforehand with the students to whom you will be lecturing. Go with the teacher or principal to the playground or cafeteria and be introduced informally to your target audience. Be friendly, but not overly so. In the Classroom.