In Arizona, individuals can receive a tax credit of up to US$200 per individual taxpayer for donations to public schools. We analyze public school tax credit donations to charter schools and document inequalities in the distribution of donations associated with the percentages of more advantaged students charter schools serve. The differences in donations may be partially attributable to parents' ability to invest in their children and the extent to which schools actively solicit donations. These dynamics suggest a Matthew effect of cumulative advantage among Arizona's charter schools. We discuss our findings in relation to a legislative proposal to change the public school tax credit program, the current political environment in Arizona, and in the context of a more transformative social change.