From an MIT study that researched the benefit of vouchers in Columbia,

Angrist et al. estimate that attending private school increased the probability of finishing eighth grade by 13-15 percentage points or 25 percent. Test scores increased by .29 standard deviations which is equivalent to about an extra year’s worth of schooling which has been estimated to increase yearly wages by 10 percent. Other markers such as teen cohabitation also improved.

Some of the authors of the above study also conducted a follow up study, writing,

The results of our follow-up study point to lasting benefits for voucher winners, with substantially higher high school graduation rates and, after adjusting for selection bias, higher test scores among those who took the ICFES exam [a college entrance test, Alex]….The size and persistence of the impact suggests PACES was a cost-effective intervention … there is substantial economic return to high school graduation in Colombia.

Hat Tip: Marginal Revolution.