This paper aims to investigate the effect of gender and nationality of 9th and 10th grade students in public and private schools of India, Pakistan and Zimbabwe on their achievement in mathematics based upon achievement data (n=5996) for session 2012-2013 which was gathered manually in 2013 from the participated schools of all the countries (India, Pakistan & Zimbabwe) where each country contributed 2000 sample of students collected from 20 schools and data of four students from Zimbabwe was lost, so total sample of the study was 5996 students. The influence of public and private schools was also examined on students’ achievement for the total sample. Data was analyzed by using t-test, analysis of variance and results of the study indicated that significant differences existed on the mathematics achievement of public and private school students of all the three countries and private school students achieved better in mathematics than public school students indicating private school students still achieve better than students of public schools. Besides, male and female students in public and private schools of each country achieved equal in mathematics indicating no gender disparity even in public and private schools which provides new insight that male and female students achieve equal in public schools as well as in private schools, although, private school students achieve more scores in mathematics than public school students. In addition, analysis of variance indicated that Indian public school and private school students achieve more than students of other two countries. Indian male students of public schools achieved more than public school students of Pakistan and Zimbabwe. Similarly, performance of Indian female students of public schools in mathematics was better than Pakistan and Zimbabwe public school students. Furthermore, both male and female students of private schools of India achieved better in mathematics than male and female private school students of Pakistan and Zimbabwe. Findings show that there was no gender disparity in public and private schools of the examined countries while nationality had significant effect on mathematics achievement of students as Indian students achieved highest in public and private schools than students of other two countries.