Поставить в предложение глагол-сказуемое в Present, Past, Future Perfect Tense в действительный залог:
On the average students attend school approximately six hours a day.

Present, Past, Future Perfect
On the average students have attended school approximately six hours a day.
On the average students had attended school approximately six hours a day.
On the average students will have attended school approximately six hours a day.

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