I remember about five years ago, when teachers were complaining about the relief classes they were getting (not that they are not complaining anymore now!) the principal at that time made a stern statement (which I will remember, God willing, till the day I retire!) that teachers do not have free periods. If a teacher teaches for six out of the ten periods per day, the teacher cannot consider himself free for the other four periods!
In fact he said, during these four periods, the teacher is required to do other teaching related work (I stress on the word work, just as he did!) like marking exercise books or exam papers, completing lesson plans, creating and producing resource materials to be used for teaching, updating the special rooms that the teacher is assigned to plus other related clerical work!