Twenty children and teenagers tested positive for the coronavirus disease 2019 (Covid-19) and 300 students are under quarantine after dance school students with virus symptoms attended a competition in central New York, county officials said.

Multiple students from a dance school in the Utica area participated in a competition Saturday, in nearby Syracuse, despite experiencing symptoms of the illness. The students later tested positive for Covid-19, Oneida County Executive Anthony Picente said at a news conference Tuesday.

“This is our largest cluster, our fastest cluster, and we’re a year into this,” Picente said.

The 20 people connected to the dance school cases who have tested positive range from 6 to 17 years old, and are from several school districts, Picente said.

The Whitesboro Central School District near Utica resumed in-person learning Wednesday, a day after going fully remote because of the exposures. Superintendent Brian Bellair told the Observer-Dispatch of Utica that he expects about 300 students from a mix of grade levels, including a few entire elementary school classes, to be under quarantine.

Picente said the dance school, which he didn’t identify, failed to practice proper pandemic protocols.

“Carelessness is preventing us from returning to normal and punishing the people who are doing all the right things,” Picente said.

Picente said he has been in touch with Onondaga County Executive Ryan McMahon in Syracuse. A spokesperson for Onondaga County told Syracuse.com that said no one had been been quarantined in that county yet.