Teaching a different Class

In science I am teaching two science classes, my class, as well as the grade 7/8 class. The first time i taught the grade 7/8 class i was very happy with the lesson and the enthusiasm that the students had. The first time i had my class for science it almost seemed like a struggle to get them to pay attention and weren't nearly as engaged. I have a few theories about this however it also was a Friday afternoon so this could have had something to do with it. After a few more times teaching i will be able to gauge whether it is the change of atmosphere/teacher or weather is it something totally different.