I believe the National Record is 388 set by Ponder High School (Texas) 2014-2015
Verified by the National Federation of High School Associations and the University Interscholastic League in Texas, Ponder High School set the national record for 3-pointers made in a season with an astonishing 388 makes.
The Lions were incredibly efficient from behind the 3-point line this season, sinking 388 of 1122 attempts (34%). They were led by shooting guard Chase Harris, who made 124 of 319 attempts over the course of the season (39%). Not far behind was point guard Brady Anderson who sunk 114 shots in 307 attempts (37%). Their best team performance came in a game on February 15 against Callisburg in which they went 21 of 45 (47%) from 3-point land. Harris put up 43 points in that game and made 8 three’s. Anderson added 34 points and 9 three’s.
http://www.krossover.com/blog/2015/06/ponder-high-school-tx-breaks-national-3-point-record/