Joe Boswell and Malo Manuao also scored for Scot's College.
Te Aute, the Rotorua captain, said he was amazed to win for his school.
"It's astonishing," said Te Aute, who was also in the 2013 team that made the semi finals.
"The boys prepped well, we had a good start to the game and carried on well and it was in my mind (up 21-0) that this was going to be comfortable but Scot's College are a good side and they were always going to bounce back."
McGarvey-Black received praise from his captain for stepping up in a final which saw little of standout player Latu Vaeno.
"Ngarohi has been one of our star players this season and forms part of a great side. It's not just the seven on the paddock as it has taken all 12 to take out this competition."
Western Heights High School were the last Bay of Plenty school to win the Condor Sevens in 1998.
The girls' title was won by Hamilton GHS who edged Southland GHS 14-12 to claim a second successive title.
Tournament teams
BOYS
Viliami Tuipolotu (Hamilton BHS)
Maake Taulahi (Rotorua BHS)
Salesi Rayasi (St Patrick's College, Silverstream)
TJ Va'a (Scot's College)
Isaac Te Aute (Rotorua BHS)
Latu Vaeno (Rotorua BHS)
Connor Garden-Bachop (Scot's College)
Sanaila Waqa (Hastings BHS)
Peter Umaga-Jensen (Scot's College)
Vilimoni Koroi (Feilding HS)
Jacob Lakopo (Otahuhu College)
Jone Lasaganibau (Rotorua BHS)
GIRLS
Georgia Mason (Southland GHS)
Alena Saili (Southland GHS)
Jacqui Chamerlain (Southland GHS)
Terina Te Tamaki (Hamilton GHS)
Tenika Willison (Hamilton GHS)
Te Awatea Whitiora Te Ura (Hamilton GHS)
Violet Hapi-Wise (Hamilton GHS)
Lauren Balsillie (FAHS)
Luka Connor (Opotiki College)
Irihapeti Moore (Fairfield College)
France Bloomfield (MAGS)
Hastings Leiataua (Otahuhu College)