But it isn't just the money...
Nor do teachers with second jobs during term-time short-change their pupils on preparing lessons, marking or assigning homework, the researchers found.
A report in the latest edition of Education Economics, says that around 17 per cent of US teachers hold outside jobs during the academic year, with many more taking jobs during the summer holidays.
However, many more male teachers - 30 per cent of the total - take on second jobs than female teachers, of whom only 11 per cent moonlighted.
Men also spent more hours a week at their second job, an average of 14. 4, against the 10.5 hours for women. The range of hours for both sexes was wide, however, with men working from six to 20 hours a week at their second job, and women from four to 15 hours.
Elementary school teachers and older teachers are less likely to moonlight, and better-educated teachers, those with computing skills and those living in urban rather than rural areas all earn more in their second jobs, the researchers found.
Moonlighters gave out less homework than non-moonlighters, but reported more "instruction-related activities outside school" - work preparation and the like - than non-moonlighters.
The study concludes that "there may not be a 'typical moonlighter'," and that "most teachers who moonlight are not devoting long hours on a regular basis to a second job just to keep the bills paid". They are merely "taking advantage of unusual opportunities or seeking income on a short-term basis, perhaps to meet unexpected expenditures".
Copies of Education Economics can be obtained from Carfax Publishing Company, PO Box 25, Abingdon, Oxfordshire.