I have taught at a public high school in BC, Canada. In the school system most schools use periods such as A, B, C, D periods. If particular subjects can not be fit in the regular timetable due to small enrollments or the shortage of teachers, the school allows to offer the course outside the regular timetable namely x-period or x-block. There is no such a system in Japan but it's quite common here. Extra curricular activities are not usually placed within the timetable. Depending on the context, I would translate ～時間目. If the x-period comes after 5th period, I would translate 6時間目。

an idea