The old school is any group of people who are traditionalists: they do things the way they've been done in the past.
Old school means something close to old-fashioned, but it’s a term with more pride behind it. If someone says, “I’m old school,” they’re saying they do it like it used to be done, which they believe was a better way. In mob movies, the old school usually thinks they’re better than the younger criminals. An old school teacher might prefer not to use computers in class. This word can be used as a noun or as an adjective, and the opposite is new school.