The purpose of this study is to show the results and the transcendental conclusions about various kinds of emotions—positive or pleasant and negative or unpleasant—and their intensity as far as they are felt by students of the fifth and sixth grade of Primary Education in the area of Physical Education, comparing the emotional states according to gender and their body-mass index (BMI). The implication is to provide a new, emotional approach, which up to now has been insufficiently dealt with in educational research. The sample was composed of 786 students of the fifth and sixth grade of Primary Education from five localities in the province of Albacete (Spain). In order to assess affectivity, the version of PANAS (Positive and Negative Affect Schedule; Watson, Clark and Tellegen, 1988) was used, validated in Spanish by Sandín (2003) for use with children and teenagers (PANASN). The results showed that the scores were higher in the positive emotions than in the negative ones. In addition, the results showed significant differences among boys and girls in the Physical Education class, especially in the variables which contribute the most to the factor of positive emotions. In relation to the analysis about the differences according to the students’ BMI, it is noteworthy that in the dimension of positive emotions, all the highest average scorings are found in the students without obesity/overweight problems. With respect to the dimension of negative emotions, all the highest average scorings were found in the students with obesity/overweight problems. The results showed significant differences according to BMI among students with obesity/overweight problems and students without obesity/overweight problems in positive emotions. There were no significant differences in negative emotions. Boys as much as girls without obesity/overweight problems show more positive emotions than boys and girls with obesity/overweight problems. Intervention programmes are required, and they should include enjoyment as an essential condition for practicing sport, encouraging the development of positive affects, and diminishing the negative affects in boys and girls.