We provide a model to analyze charter school educational practices. Students differ in cognitive ability, motivation, and household income. Student achievement depends on ability, match of their school's curriculum to their ability, and effort. Charter schools choose curriculum to maximize achievement gains, optimally setting curriculum to attract lower ability students. Achievement gains are modest, consistent with empirical evidence. We also investigate "no excuses" charter schools. These charters enforce an effort minimum that attracts highly motivated students. We find, consistent with the evidence, that these charters are highly effective in increasing achievement, with the largest gains accruing to lower ability students.