The 1954 Milan High School Indians won the Indiana High School Boys Basketball Tournament championship in 1954. With an enrollment of only 161, Milan was the smallest school ever to win a single-class state basketball title in Indiana, beating the team from the much larger Muncie Central High School...
https://en.wikipedia.org/wiki/1954_Milan_High_School_basketball_team
beat Oscar Robertson's team in the quarterfinals. Wikipedia says they won the championship game 32-30 by stalling for 4 minutes in the 4th quarter before their star Plump hit a jumper for the winning points.
The year prior, Milan had made it to the state final four before getting blown out by a South Bend team. For the 1954 team that won the state championship, they returned 4 starters from the team that a year earlier had made it to their final four.