But it was a spark for Hastings, who won both the Cup and the national Top Four title with a 27-15 win over Auckland school King's College last year.
With a penalty just before the break and returning firing on all four after halftime, the home team and cup holders were rewarded with a try to hooker Jacob Dorward after about seven minutes.
Converted, the try quickly made it 10-all but Napier regathered the ball as the kickoff was misfielded, a turn of events which enabled the pressure on Hastings which led to the 17-year-old Robinson's ultimate clincher.
The match doubled as a second-round game in the Super 8, a central North Island schools championship. Napier's first defence of the Cup will be in Napier next Saturday against Tauranga Boys College.
Result:
Moascar Cup, at Hastings: Napier Boys High School (challengers) 15 (Jayden Stock, Will Robinson tries; Carlos Kemp pen, con) beat Hastings Boys High School (holders) 10 (Jacob Dorward try; Hoera Stephenson pen, con). Halftime: Napier 10, Hastings 3.