Mr Robertson (42) was greeted on his arrival by current head boy Will Meiklejohn (19) and prefect Liam Taylor (17) to be told the latter recently joined the Labour Party so he could vote for Mr Robertson in the leadership contest.
After some backslapping and shaking of hands, Mr Robertson told Liam he was thrilled with the news.
Getting time for an interview with Mr Robertson at the school was problematic as he was in demand from teachers, who wanted to shake his hand and wish him the best for the leadership contest.
Inquiries by the Otago Daily Times to those teachers on what Mr Robertson was like as a pupil were met with firm ''no comments'' and lots of laughter.
Mr Robertson, Andrew Little, interim leader David Parker and late-entry Nanaia Mahuta are contesting the leadership in a primary-style campaign, which will see the MPs address party members throughout the country from next week.
The Wellington Central MP believed he was the one who could unite what seems to be a divided caucus and prepare the party to become the government in 2017.
But the MPs first had to stop talking about themselves and start talking about the things that mattered to New Zealanders, he said.
''We need simple, practicable and believable ideas. We need unity on ideas which will resonate with the public.
''The public struggled to get a handle on who we were. We need to change our message and talk directly to New Zealanders about the things that matter. Once we get a consistent message, people will come back to us.
''No-one underestimates the challenge - least of all me.''
One of the promises Mr Robertson did make yesterday was the leader elected by the caucus, party and union affiliates on November 18 would need to lead the party for the next three years.
He would not make another challenge if he was unsuccessful next month until the three-year term was up.
He believed the other Labour MPs would also stand by the decision on November 18.
