A child should learn to be observant while at school. It's possible that this individual is the son or daughter of one of the teachers or officials at the school. Equally important, it is also possible that this person doesn't belong on the school grounds in the first place. While at school, it is a good idea that your child NOT be alone in a classroom with a teacher. It is best to have them bring a friend along if they need to speak with the teacher, or if the teacher wants to see them, such as after school or during a scheduled break.
Some schools require their students to wear uniforms.
Other schools do not. As a parent, be aware of the type of clothing your child is wearing to school. Be careful of such items as jackets with professional sports insignias and other items that are used as gang identifications.