The phrase is perfectly clear and understandable.
I would choose "break" or "leisure" depending on the amount of time involved.
- I would use "break" for short periods, times when the activity stops (to provide a rest, or to allow moving to another activity), but which is not long enough to do something else. So there is a break between classes, but students are not "at leisure" because the interval is too short to start a new activity.
- I would use "leisure" or "at leisure" to refer to a longer period of unscheduled time, long enough to do some activity of one's choice -- read, bicycle, do laundry or chores, play sports, sleep, write letters, etc.
That means that you might use both words for different times in the same day:
8:00 - Breakfast
8:30 - Clean the area
9:00 - First classroom instruction period
9:55 - Break between classes
10:00 - Second classroom instruction period
12:00 - Lunch
1:00 - Afternoon at leisure
5:00 - Dinner
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