What Harm Does a Hood Do?

Something that grinds my gears is when teachers tell us to put our hoods down. Sometimes I am having a bad hair day and really want my hood to be up. Sometimes I am sad and don’t want people talking to me and if I have my hood up, then people will leave me alone. Outside of school, I always have my hood up unless I am having a good hair day and I wish I could have it during school. I don’t understand the point of no hoods or hats allowed. What’s the harm in having a hood on?