Select from list and add if not found field

I would like to create a drop down list for schools, but I don't want to enter all schools. I want users to look at the list first, if they can't find their schools, they can add it to the list and the school they added are showed in the list. So the users registered after them can select their school from the list. Is there a way to create that type of field? I think it is like free tagging.