Just because of that line, the whole comparison between the first stanza, which illustrated our innocent and healthily imaginative childhood, and the second, which illustrated a stricter and guided society, had changed. It caused that first stanza to look short-lived and the second one to be incessant, everlasting, and without end. This poems message, implied greatly by the last line, tells us that we must enjoy our childhood and never take any opportunity or moment with our loved ones for granted. It also tells us that although we have our chance to have an occasional splurge, order, law, and justice will be prevalent at all times. This holds true to all people- especially those in First Grade.
I think the main theme is the flawed school system. I dont mean the school system in a particular area or country. Hes talking about school in general and how it kills the individuals creativity and intellectual independence. He mentions how his imagination was wild and free before first grade: Until then, every forest had wolves in it and we could talk to water. But when he enters first grade, hes faced with drab reality: his teacher is a woman with the gray breath; hes saying that the state of mind the school creates in you is something that youre stuck with for the rest of your life.