According to a US federal Department of Education survey released on Tuesday, 94 per cent of public school teachers in the United States reported paying for supplies without reimbursement in the school year that straddled 2014 and 2015.

It made little difference whether they taught in cities, suburbs or rural areas, or whether or not their students were poor — virtually every public school teacher said they had used their own money for their classrooms.

"It’s almost expected, especially in the summer months creeping up into September," Mr. Yung said. "It’s just something we kind of naturally do."

The teachers who reported spending their own money on supplies shelled out $479 (£406) each on average, according to the survey. Seven percent reported spending more than $1,000.

The findings, based on a nationally representative sample of tens of thousands of teachers, underscore the demands teachers across the country have been making in recent months amid protests over stagnant pay and underfunding.

The latest took place on Wednesday in North Carolina, where some schools cancelled classes as throngs of teachers and others marched in the capital. Similar rallies have been held this year in Arizona, Kentucky, Oklahoma and West Virginia.

The protesters have been successful at extracting concessions from conservative lawmakers, though the deals have not always fully met their demands.

On average, public school teachers earned just under $60,000 (£50,862) last school year, according to the National Education Association, but pay is so low in some areas that officials have been recruiting overseas.

Limited budgets and red tape have led some teachers to seek outside funds for classroom projects. Like Mr. Yung, some of them use DonorsChoose.org, a crowdfunding website where educators can solicit donations for supplies, trips, and other projects.