The purpose of this study was to investigate high school students’ mathematics achievement. Their mathematics achievement was determined by their latest mathematics Grade Point Average (GPA) results that were collected at the end of the first term of 2019-2020 academic year. Level of sources of mathematics self-efficacy, gender and school types were examined to gain information about their mathematics achievement by using descriptive research design. Descriptive statistics were used to analyze students’ mathematics achievement in terms of level of sources of mathematics self-efficacy, gender and school types. The participants consisted of 9th, 10th and 11th grade students. A total of 644 students from private and state schools took part in this study. The Turkish version of the sources of mathematics self-efficacy survey developed by Perkmen (2016) was used to measure students’ sources of mathematics self-efficacy. It was investigated whether there was a correlation between students’ level of sources of mathematics self-efficacy and mathematics achievement or not. The results showed that there was a statistically significant and positive correlation between students’ mathematics achievement and mastery experiences, vicarious experience, social persuasion, and psychological states. Also, independent sample t-tests were used to examine gender difference and school types in terms of students’ mathematics achievement. There was a statistically significant mean difference between female and male students in favoring of female students’ mathematics achievement with a medium effect size. Furthermore, there was a statistically significant mean difference between private and state schools with a small effect size in favoring of private high school students’ mathematics achievement.