Students do not know when a reinforcer is coming, but know that if they continue to do their work, it will eventually appear. the students are being reinforced on what type of schedule?

Based on the scenario, the students are being reinforced on variable ratio Schedule.
A variable ratio schedule is a form of scheduling in which responses are created after being exposed to a number of unpredictable response. Since the students need to be on their toes all the time, this type of scheduling will provide the highest rate of student response