The need for developing critical thinking (CT) has been broadly discussed and its importance acknowledged in a myriad of disciplines. This quantitative study attempts to investigate the level of critical thinking skills as well as the impact of grade level, gender, and nationality on the development of these skills among 279 Bosnian-Herzegovinian and Turkish high school students. The instrument used for this research is a survey questionnaire which consists of 17 items made up of four subscales: Convictions that inhibit critical thinking development (CICTD), Application of critical thinking (ACT), Class activities that prevent critical thinking (CAPCT), and Teacher support for critical thinking development (TSCTD). The findings reveal that students' nationality and grade level significantly impact critical thinking development, whereas students' gender does not have a significant effect on the development of CT skills. These findings are widely applicable as they can be used by language teachers and teachers of general courses to contribute to their students' critical thinking development by structuring their syllabi accordingly. The findings also point to an urgent need to revise the existing curricula and design more adequate ones which would include a greater number of activities fostering critical thinking skills.