This is the 5680th most frequent Japanese word.
せっせと
Diligently; industriously; busily.
The word 'せっせと' is used here to emphasize the continuous and earnest efforts of working.
彼はせっせと働いている。
He is diligently working.
In this context, 'せっせと' shows the enthusiasm and focus of the students while completing their assignments.
学生たちはせっせと宿題をしている。
The students are busily doing their homework.
The usage of 'せっせと' describes the meticulous and energetic manner of the gardener's actions.