The official GNS monitoring service confirmed the volcano erupted shortly after 1:20 p.m. Wednesday, New Zealand time, and it was witnessed by 90 students and four teachers from Tamatea Intermediate School, Stuff.co.nz reported.
Teacher Lomi Schaumkel said the group was just half a mile from the crater when the eruption began and they captured it on video.
"We were right up next to it," Schaumkel said. "It was just amazing. It was pretty scary from where we were and it just looked absolutely spectacular, the ash that came out. It really did look like one of those atom bomb explosions, and it made a rumbling sound."
Mount Tongariro became active in August after lying dormant for more than a century.
Wednesday's eruption lasted about five minutes, GNS said, but there was "significant emission of ash" into the atmosphere.
Civil Defense warned people to stay indoors if ash is present and keep all windows closed.
A woman who lives about a half mile from the eruption site reported a black cloud of ash over her house and the smell of sulfur in the air.