NSW Teachers Federation members at Singleton High left school grounds when Minister for Education Sarah Mitchell visited on Friday.
Federation regional organiser Jack Galvin Waight said about 40 members "took industrial action and walked out", frustrated at the government's "refusal to act on workloads and non competitive salaries" and subsequent teacher shortages.
"They actually turned their backs on the minister as she walked into the school grounds, such is the disdain they feel over the damage the state government is doing to the profession and students' learning," Mr Galvin Waight said.
He said there are 37.3 vacant permanent positions across 44 schools in the Upper Hunter, including six at Muswellbrook High.
"Not to mention the daily shortage of casual teachers that means schools are having to split classes or students are forced to learn on their own with minimal supervision."
A Department of Education said the union's figures were incorrect.
"The number of vacancies in the Hunter is low, with an average of less than one vacancy per school," the spokesperson said.
"The majority of schools in the region have a high number of staff employed above what they are allocated, which they are able to do thanks to record school funding."
Ms Mitchell said it had been a "pleasure" to visit the school and hear feedback from Upper Hunter principals and teachers about the department's Rewarding Excellence in Teaching reform, "which could see expert classroom teachers attract salaries of up to $147,000 per year, in recognition of their skills and impact".
"It's vital we hear from as many teachers as possible as we develop this innovative policy," she said.
"I am so grateful to teachers from the Upper Hunter area for sharing their valuable input - it was so positive to sit around the table and have a constructive discussion about these ideas.
"I'll continue doing my job by visiting schools and talking with students, parents and teachers."
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