The study investigated the validity and reliability of Osborn’s scale and also examined the influence of gender, class and age on the scale’s reliability with a view to developing a student-friendly method of determining test validity by classroom teachers. The population for the study comprised senior secondary school students in Kwara State. The sample for the study consisted of 450 students and 30 school heads. Three Local Government Areas (LGAs) were selected from the state. Ten secondary schools were randomly selected from the LGAs. These were 450 students and 30 heads selected from these 10 secondary schools. They were selected using purposive sampling technique. A questionnaire eliciting information on respondents’ personal data was developed to accompany the 31 items of the Osborn’s scale and used for data collection. The responses of 389 students and 30 heads that completed the questions and rating scale were analyzed using appropriate statistics The results showed that the Osborn’s scale is valid for use in measuring subject difficulty among the school students in Kwara state with a concurrent validity of 0.89 and Cronbach’s alpha of 0.55. All subjects were rated as either not difficult or just difficult. Mathematics was rated by secondary school students as the most difficult subject while religion was rated the least difficult. Among the science subjects, Agricultural science was rated as the least difficult subject. Generally, science subjects were rated more difficult than arts and commercial subjects. Moreover, gender was significantly related to students’ ratings in Mathematics and Economics while it had no significant relationship with students’ ratings of Physics, English, Biology, Geography, Agricultural Science, Chemistry and Religion. In conclusion, the Osborn’s scale is valid and effective for determining the difficulty of school subjects and is suitable for use in Nigerian secondary schools.