2010-11: William Lagesson played for the HK Kings U18 team, a lower division club on the West Coast of Sweden, and skated for Goteborg 2 in the TV-Pucken tournament for high school players. He had 2 assists in eight games for Goteborg 2.
2011-12: Lagesson moved up to the top division of Swedish junior hockey; skating for Frolunda’s U18 and U16 teams. He was a team captain for Goteborg at the TV-Pucken tournament and played for Sweden’s U16 team. Lagesson had 3 assists and was +6 with 2 penalty minutes in 18 games (including two playoff games) with Frolunda’s bronze medal-winning U18 team. He was +5 with no points nor penalties in five games with Frolunda’s U16 squad. He scored one goal in three international games with the Sweden U16 team and was +8 with 2 goals and 4 penalty minutes in eight games for Goteborg in the TV-Pucken tournament.
2012-13: Lagesson played eight games for Frolunda in Sweden’s U20 SuperElit league and was an assistant captain for the club’s U18 squad. He played for Sweden’s U17 team at the 2013 World Hockey Challenge. Lagesson was +4 with no points or penalties in six regular season games and two playoff contests in SuperElit play. He scored 4 goals with 15 assists and was +17 with 74 penalty minutes in 32 regular season games for the Frolunda U18 squad. He scored 1 goal with 2 assists and was +4 with 4 penalty minutes in three playoff games. Lagesson had 1 assist with 12 penalty minutes in six games as Sweden captured a gold medal at the 2013 WHC in Canada.
2013-14: Lagesson played 47 games, including three playoff contests, for Frolunda’s U20 team in his first SuperElit League season and played for Sweden’s U18 team in the 2013 Ivan Hlinka Memorial and 2014 World Junior Championship tournaments. He scored 8 goals with 12 assists and was +27 with 30 penalty minutes in 44 regular season games and had 1 assist and was +1 with 2 penalty minutes in the playoffs. Lagesson played for Frolunda’s U18 team, He scored 2 goals with 2 assists and had 10 penalty minutes in five playoff games for the bronze medal squad after registering 1 assist and finishing +4 in four regular season games. Lagesson scored 1 goal with 1 assist and was +3 with 2 penalty minutes in four games at the Ivan Hlinka tournament and was +5 with 3 goals and 6 penalty minutes in seven games at the WJC. Sweden lost to Russia, 3-1, in the bronze medal game. Lagesson was ranked 18th amongst European skaters in Central Scouting’s final rankings prior to the 2014 NHL Draft.