Pretty sure I've posted this before, but I'm not going to dig for the exact statute. In the late 1960's the state passed an education reform bill that required all independent school districts to operate a secondary school. So smaller districts that only had primary schools either had to merge or open a high school. In most cases, this had already happened (the large suburban districts). The biggest example of this was the creation of the St. Michael - Albertville district - the last new ISD created in the state other than mergers and consolidations.
There are a few exceptions today. There are some border communities where the high school has since closed, like Browns Valley, where the kids are sent to school in another state (SD in this case).
There remain two non-operating school districts in Minnesota, and I don't know why they were exempted from the law. One is Prinsburg, which is a small, very conservative town where virtually everyone goes to the church school. The few that do not are bussed to MACCRAY, and the Prinsburg non-op school district has to pay for it.
The second is Franconia, which is between Scandia and Taylors Falls on the St. Croix river. Those kids, few as they must be, go to Osceola, WI. I have no idea why that hasn't been absorbed by either Chisago Lakes or Forest Lake.