This is actually a hugely significant moment in the novel that marks real progression in terms of the narrator's battle to reach his students and to teach them. Having had so many problems with them and having had to be very strict, he finally sees that they are beginning to respond to him, but not just out of fear or dislike, but through a genuine appreciation and understanding of what he is trying to do in teaching them, as the gift of flowers that lies on his table demonstrates. Note how the narrator describes these flowers:
In the centre of my table was a large vase in which was neatly arranged a bunch of flowers. Some were slightly bedraggled; all had evidently been collected from the tiny backyards and window boxes of their homes. For me this was the most wonderful bouquet in the world; it was an accolate bestowed collectively by them on me.
Note how, in spite of their bedraggled appearance, the narrator sees these flowers for what they represent: an "accolade" given to him by them, which explains why he is able to say thank you with such a "full heart." He appears to have finally gained their confidence, respect and love.