We have conducted field measurements and a questionnaire survey, and statistical analysis of the results, in order to review applicable criteria for assessing the thermal environment in school classrooms without air conditioning. From survey results for both summer and winter, we found that the indoor-outdoor temperature difference is more strongly correlated with students' thermal sensations in classrooms than the predicted mean vote index: the correlation coefficient is 0.979 in summer and -0.992 in winter. These results indicate that the indoor-outdoor temperature difference is a valid criterion for assessing the thermal environment of classrooms. In addition, taking the indoor-outdoor temperature difference as the explanatory variable and the index of the students' thermal sensations as the objective variable, we performed a logistic regression analysis. As a result, formulas for predicting students' thermal sensations in classrooms were derived. These findings should be beneficial in establishing an appropriate learning environment for students.