Titles of poems are the first piece of the text readers come across. Therefore, the title of a poem is meant to draw a reader into the poem. Sometimes, the title is also meant to make the reader think about how the title of the poem speaks to what the poet wishes the reader to consider while reading the poem itself. Poem titles which are ambiguous or seemingly confusing are meant to make the reader question the poet's titling of the poem.
The title of Rabindranath Tagore's poem "The Flower School" draws readers in given the "fact" that flowers simply do not go to school. Therefore, an engaged reader must consider what school could have to do with flowers. Perhaps, the poet is stating that flowers have something to teach mankind.
Given that knowledge is one of the things that gives mankind power ("knowledge is power"), the idea that nature, more specifically flowers, could enlighten mankind is curious. The speaker questions the act of learning by the flowers, comparing them to the speaker's (assumed) own experiences in school: "if they want to come out to play before it is time, their master makes them stand in a corner." Here, the speaker draws the reader into seeing a comparison between the flowers and himself or herself. The flowers are not much different than the schoolchildren of the world. This is the lesson the flowers have. The flowers have "schooled" those reading the poem. The flowers, figuratively, run the school.
Therefore, the "Flower School" is meant to refer to the knowledge that nature (symbolized by the flowers) possesses. Nature wishes to educate the youth of the world (given the setting is that of a school) on the importance of nature and the similarities that nature has to the youth. This final comparison is illustrated in the final line: "they have their mother as I have my own."