(CNN) Almost all public school teachers -- 94% -- have spent some of their own money on school supplies without reimbursement, according to a survey done by the National Center for Education Statistics. The data was published on Monday and collected from 2014 to 2016.

On average, public school teachers spent $479 of their own money during two years. The median amount spent was $297. However, the plurality (44%) of teachers spent $250 or less.

The amount of money spent depended on the school classification, community type, instructional level and the socioeconomic status of the students. Teachers who worked at charter schools were less likely to spend their own money, but the majority of charter school teachers (88%) did say they had spent their own money and they spent about the same amount as those in public schools.

There was a direct relationship between the amount of money a teacher spent and the portion of students they taught who were approved for free or reduced-price lunches, an indicator of their socioeconomic status.

In schools where 75% or more of the students participated in free or reduced-price lunches, teachers were spending, on average, $554 on school supplies. In schools with 0-34% of students approved for free or reduced-price lunches, teachers were spending an average of $434 on school supplies.

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