2009-10: Fredrik Larsson played 15 games for the Brynas U18 and U16 teams and skated for Gastrikland in the TV-Pucken tournament for high school-aged players. He had 1 assist with 8 penalty minutes in his limited action with the Brynas squads and in eight games for Gastrikland had 2 assists and 6 penalty minutes.
2010-11: Larsson played in 41 games including five playoff contests in his first full season with the Brynas U18 team. In 36 regular season games he scored 6 goals with 18 assists and was plus-48 with a team-leading 73 penalty minutes. Brynas finished first in Allsvenskan U18 North and reached the semifinals in the playoffs. In five playoff games Larsson was plus-one with no points and 6 penalty minutes.
2011-12: Larsson appeared in 16 SuperElit league games with Brynas U20 and led the Brynas U18 team in penalty minutes for the second season in a row. In 14 regular season games for the third-place Brynas U20 team he scored 1 goal with 3 assists and was plus-two with 24 penalty minutes. In the two-game playoff series with Sodertalje he was minus-one with 6 penalty minutes. Larsson racked up 158 penalty minutes in 35 game with the Brynas U18 team but was plus-33 with 3 goals and 12 assists. Brynas finished fourth and reached the U18 playoff semifinals. In five playoff games Larsson scored 1 goal and was plus-one with 8 penalty minutes. Not among the 120 European skaters in Central Scouting's final rankings prior to the 2012 NHL Draft he was selected by Philadelphia in the fourth round (111th overall).