Schaefer became a rarity before and after the 2025 NHL Draft.

Not many players -- none in the 21st century -- have held the No. 1 spot on NHL Central Scouting's final ranking of North American skaters having played fewer than 20 games in a season. Schaefer did. Since 2000, four defensemen had been chosen with the No. 1 pick in the NHL draft: Owen Power (2021, Buffalo Sabres), Rasmus Dahlin (2018, Sabres), Aaron Ekblad (2014, Florida Panthers) and Erik Johnson (2006, St. Louis Blues).

Schaefer became the fifth when he was selected by the New York Islanders in 2025. This came after he missed the opening nine games of the 2024-25 season because of mononucleosis and dealing with the death of Erie Otters owner, mentor and friend Jim Waters on Dec. 13, 2024. Then Schaefer's season ended Dec. 27 because of a broken clavicle sustained while playing for Canada at the 2025 IIHF World Junior Championship in Ottawa. He had 22 points (seven goals, 15 assists) and was plus-21 in 17 games with Erie of the Ontario Hockey League that season. His average of 1.29 points per game was third among OHL defensemen to play at least 15 games.

Schaefer had an assist in a 4-3 loss at the Pittsburgh Penguins on Oct. 9, 2025, to become the youngest defenseman in NHL history (18 years, 34 days) to have a point in his debut, previously held by Scott Niedermayer (18 years, 46 days on Oct. 16, 1991). He scored his first NHL goal in a 4-2 loss to the Washington Capitals at UBS Arena on Oct. 11, making him the second-youngest at his position with a goal. Ross Johnstone scored at 17 years, 207 days for the Toronto Maple Leafs in 1943-44, then scored again later that season at 17 years, 328 days.

Schaefer finished his first month in the League with eight goals (three goals, five assists) in 11 games to earn NHL "Rookie of the Month" honors for October. His season-opening point streak of six games (two goals, five assists from Oct. 9-21) tied Marek Zidlicky of the Nashville Predators (Oct. 9-23, 2003: 2-5—7 two goals, five assists from Oct. 9-23. 2003) for the longest such run by a rookie defenseman in NHL history.

He began November by scoring twice in a 3-2 win against the Columbus Blue Jackets on Nov. 2 to become the youngest defenseman in NHL history with a multigoal game. It was his first at 18 years, 58 days, besting Bobby Orr (18 years, 248 days on Nov. 23, 1966). With five goals and five assists, Schaefer also joined Phil Housley (seven games in 1982-83) as the only 18-year-old at his position in the League to reach the 10-point mark through 12 games of a season.

Schaefer scored twice in a 5-4 win against the Florida Panthers on March 1, 2026, to become the youngest defenseman in NHL history (18 years, 177 days), and the sixth-youngest skater, to score 20 goals in a season. He was also the third-youngest at his position to have a three-point game, behind Craig Wolanin (18 years, 135 days) and Doug Bodger (18 years, 151 days). The performance ended a week when he earned NHL First Star honors. He then passed Housley (57) with his 58th point in a 4-3 loss at the Carolina Hurricances on April 4, the most by an 18-year-old defenseman in NHL history.

Schaefer won the 2026 Calder Trophy given to the NHL rookie of the year, becoming the youngest winner in NHL history, one day younger than Colorado Avalanche forward Nathan MacKinnon in 2013-14 (18 years, 224 days), and the first unanimous vote (198 ballots) since Teemu Selanne of the Winnipeg Jets received first-place votes on all 50 ballots after his historic 76-goal season in 1992-93. He led rookies in time on ice per game (24:41), tied Anaheim Ducks forward Beckett Sennecke for the lead in goals (23) and was third in points (59), the latter setting an NHL record by an 18-year-old defenseman. Schaefer also tied the single-season NHL record for most goals by a rookie defenseman and became the first at the position in more than 90 years to lead rookies in goals (tied or outright).

Schaefer had five points (one goal, four assists) in seven games and won a gold medal at the 2024 World Under-18s and another at the 2024 Under-17 World Hockey Challenge, when he was Canada's captain, after he had four points (one goal, three assists) in eight games. At the Hlinka Gretzky Cup, he helped Canada win gold with six points (two goals, four assists) in five games as team captain.

In the 2023 Canada Winter Games, Schaefer had six points (three goals, three assists) in six games and scored at 7:59 of double overtime to give Team Ontario a 3-2 win against Team Saskatchewan in the gold-medal game.