This study examined the high schools' state tests performances in mathematics, reading, and science of an open-enrollment STEM-focused charter school system,Harmony Public Schools(HPS), between 2010 and 2013, and compared them with the performance of matched traditional public schools (TPS) in Texas. After propensity score matching, 12 HPS schools were compared with 32 matched public schools. Independent sample t-tests were performed to compare the schools' TAKS and STAAR scores in each year. One-way ANOVA was conducted to examine differences across years for each school type and Univariate General Linear Model was used to investigate if school type by year interaction had an effect on scores for each subject over the years. T-test results revealed that HPS scores were significantly higher than TPS scores in most of the categories. One-way ANOVA results showed that both TPS and HPS scores dropped significantly in most of the categories during the transition from TAKS to STAAR and univariate GLM analyses indicated significant interaction effects between school type and year variables for all but 10th grade mathematics.