Johnstone (17), playing for the Kings High School First XI in its interschool match against Southland Boys High School, hit 167 on Monday, the highest score by a Kings player in an interschool fixture.
The previous highest was 156 by Gerry Stewart in 1954 against Otago Boys.
Other players to have scored centuries in interschool matches include some big Otago sporting names.
Former Otago captain Warren Lees scored 136 against Shirley Boys in 1968 and 147 against Otago Boys in 1969.
Ian Rutherford scored an unbeaten 101 against Southland Boys in 1974, and former New Zealand basketball captain Glen Denham hit 100 against Shirley Boys in 1982.
Brendon McCullum, who attended Kings, did not score a century in interschool games, though he did get 94 against Southland in 2000.
Johnstone's knock was all the more impressive as he had the flu and was still feeling a bit crook when he went to the wicket.
He batted for more than four hours, hitting 26 fours and one six.
He opened the batting, facing the first ball, and was the seventh batsman out, having scored 63% of his side's runs when dismissed.
The year 13 pupil was dropped once, in the slips, when he was on 107, and was eventually out caught at first slip.
It was his third century, and by far his highest score.
Kings won the match on the first innings after scoring 328, to which Southland Boys replied with 236 for nine declared in its first dig.
Kings scored 134 for eight declared in its second innings before Southland Boys, needing 227 to win in its final turn at bat, ended up on 168 for eight.
