Parents who volunteer, or who lobby for improvements in school quality, are generally seen as providing a school-wide public good. If so, straightforward public-good theory predicts that free-riding will reduce average involvement at larger schools. This study uses longitudinal data to follow families over time, as their children move from middle schools to high schools, thus netting out unobservable differences among families. Increases in school size result in significant reductions in parental involvement, although the magnitude of the effect is small. If parents experience a doubling in school size, they are 2 percentage points less likely to increase their contacts with the school, and 5 percentage points less likely to increase their volunteering. A continuous-treatment propensity-score method tests whether the results are driven by selection into treatment. The parental contact results are robust to this test, while the volunteer results are not. Also, there is some evidence that parents see their involvement as a substitute, rather than a complement, for perceived school quality. (Contains 9 tables and 2 figures.)