Mwm,
We have only had it off for a few years now. Actually the kids have had it off longer but not the teachers. Prior to MLK day becoming a federal holiday, we always had in-service days on that Monday. When it became a holiday, they started telling the kids they had off for the holiday, of course, teachers still had their in-service day.
As a teacher it upset me and others that we were telling the kids they had off for the holiday but we were still working. They should have just told the truth, it was in-service same as always. Eventually the district started giving us off as well, I think they had heard enough complaints and our union bargained it into our contract.
On another note though, I swear we are the only district (or state because I know as a child we never had off either) that goes to school on President's Day.