He said both he and the rest of the team gained confidence from playing in the tough US tournament.
''It was good to see how we matched up against players from a country where volleyball is one of their greatest sports. I think we all learned a lot overall and I think we came back as a better team,'' he said.
As well as top age group state teams, there were national teams from Canada, the US and Australia.
There were also benefits for Johnstone on an individual level, and he has been approached by coaches from several US colleges.
''Going over there obviously increased the exposure,'' he said.
Still in Year 12, Johnstone said he could potentially take up a scholarship, but he also wanted to focus on his schoolwork to make sure he had other options.
His goal for 2017 is to make the New Zealand Under 19 team to play in next year's Asian Championships.
The under 20 team finished in fifth place in Myanmar, beating the Maldives in the fifth and sixth playoff.
Year 13 student Michael was a first time New Zealand indoor representative, and said he felt playing in the tournament helped him change his skill set and the way he played.
''It was inspiring to see how well other countries trained their team - some of their volleyball play was amazing,'' he said.
As the senior volleyball season winds down, the junior season begins, and for the beach volleyballers, it's time to hit the sand, with Michael and Gillespie hoping to build on their New Zealand secondary school title success.
''I'll be look at jumping onto the summer pro tour and entering all the local tournaments leading up to the pro tour,'' said Michael.
Michael was also part of the Western Heights team that won the New Zealand secondary schools indoor title for the 10th time in 2015.
Western Heights volleyball coach Eugene Bogun said it was disappointing to finish sixth at this year's tournament, which was won by Westlake Boys' High School.
''But we won our fifth Bay of Plenty title in a row and that's quite outstanding considering usually the top teams in New Zealand come from the Bay, so that's a good achievement, but we are a bit disappointed at national level.''
Bogun said the difference in size of this year's five New Zealand rep players showed there was no one body shape that made a good player.
''Jon Newman is quite short, but he's very agile and quick on the court and willing to put his body on the line. The others four boys are quite big - they are 6ft 4, 6ft 5 - and very athletic and quick on the court.''
But at the root of their success was hard work, said Bogun.
''They work hard, they have a good work ethic and are always wanting to improve together. They work well in the classroom as well, they are good, all-round students.''
