The tone in this poem shifts. In the beginning, the speaker's tone is one of frustration. The speaker (teacher of the class) describes his frustration in being unable to engage his students. At first, the speaker suggests that the students are underwater while he is on shore, alone. The bright ripples of the blinds represent the ripples on the surface of the water. The speaker/teacher is figuratively on shore while he tries to communicate to the boys underwater: a difficult task.
However, the students gradually engage with the teacher more and more. Each student first gives him a pondering look and then a look of acknowledgment:
And then he turns again, with a little, glad
Thrill of his work he turns again from me,
Having found what he wanted, having got what was to be had.
The teacher starts to get through to the students, "the stream of awakening ripple and pass / From me to the boys." They begin to learn. This moment of mutual recognition is the "best of school." Lawrence uses provocative imagery of the boys beginning to cling to him like tendrils. He means to illustrate how they are more and more interested in what he has to teach. In the end, his initial frustration is gone. His tone is now joyful. He is celebrating the mutual appreciation of teaching and those being taught: "their thrills are mine."