2009-10: Beattie was one of the leading scorers for New Jersey's Pingry School as a junior and played club hockey for the New Jersey Renegades in the Metropolitan Junior Hockey League. He scored 34 goals with 33 assists for Pingry in 25 games. Pingry was undefeated in its conference and reached the Prep B Championship game and New Jersey Non-Public School tournament quarterfinals. In 42 games for the Renegades, Beattie scored 6 goals with 10 assists and garnered 60 penalty minutes.
2010-11: Beattie was one of the top scorers in New Jersey high school hockey in his senior season at Pingry and had 47 points in 38 games for the New Jersey Junior Titans in the MJHL. Beattie led all Skylands Conference forwards with 90 points. Pingry reached the finals of the Prep B tournament, falling to Princeton Day School, and lost to top-ranked Delbarton in the quarterfinals of the Non-Public School tournament Skating for the Junior Titans, Beattie scored 23 goals with 24 assists and had 34 penalty minutes. He scored 2 goals in two playoff games.
2011-12: Beattie spent a post-graduate season at Phillips Exeter in Massachusetts and committed to playing college hockey at Yale in 2012-13. Beattie was the second-leading scorer in New England prep school hockey with 39 goals and 34 assists in 28 games. Beattie participated in the prestigious Beantown Classic tournament following the season. Exeter, seeded third going into the prep tournament, lost to second-seeded Nobles and Greenough in the semifinals; finishing with a record of 22-3-5. Ranked 108th amongst North American skaters in Central Scouting's final rankings, Beattie was selected by Vancouver in the seventh round (207th overall) of the 2012 NHL Draft.