At high school level, Du Plessis attended and played rugby for Paarl Gimnasium. He was included in the Western Province squad for the 2015 Under-16 Grant Khomo Week, only to withdraw through injury. In the same year, he was selected in the South African Rugby Union's High Performance program.[5]
In 2016, he was named in Western Province's squad for the Under-18 Craven Week tournament held in Durban.[6] He started all three matches, scoring two tries in their second match against the Sharks,[7] and helping them to a 27–20 victory over the Golden Lions in the main match[8] to be crowned unofficial champions.[9] At the conclusion of the tournament, Du Plessis was included in the South Africa Schools squad for the Under-19 International Series,[10] helping the team to victories over Wales,[11]France — scoring two tries in a 42–3 win[12] — and England.[13]
Du Plessis captained Paarl Gimnasium's first team during 2017,[5] and was again included in the Craven Week side for the tournament in Johannesburg.[14] He started all three matches at the tournament, scoring tries in matches against the Pumas[15] and the Free State Cheetahs.[16] He received a second call-up to the South Africa Schools squad for the 2017 edition of the Under-19 International Series,[17] but could not prevent the team suffering defeats to Wales,[18] France[19] and England, with Du Plessis scoring two tries in the final match in a 22–52 defeat.[20]
Shortly after returning to South Africa, Du Plessis was called up by the senior squad to travel to the 2018 Hamilton Sevens as their reserve player,[22] but a pulled hamstring resulted in Du Plessis being a late withdrawal from the team.[23] This setback proved to be minor, as he was included in their next squad for the 2018 USA Sevens,[24][25] where he duly made his debut. He also appeared at the events in Canada (where he scored his first try in the series),[26]Hong Kong (where he scored 7 tries for the team)[27] and Singapore,[28] in a season that saw South Africa go on to win the series by pipping Fiji to the title.[29]
Du Plessis returned to the fifteen-man version of the sport when he was called up to the South Africa Under-20 squad for the World Rugby Under 20 Championship held in France.[30] He did not feature in their opening match of the tournament against Georgia, but was named in the starting lineup for their second match against Ireland. He scored a try in the 19th minute of the match to set his team on the way to a 30–17 victory.[31] However, he was replaced at half-time, and a gluteal muscle injury ruled him out of the remainder of the championship.[32]
He returned to domestic action in South Africa for the Sharks in the Under-19 Provincial Championship.[33]
