The poem says that "flowers go to school in underground schools" and "they do their homework behind closed doors" both express that soil is a school.
The flower's mother is rain, and the content expressed in the poem is also obvious: "When the rain comes, they will have a holiday", which means that when the rain mother comes, the flower will break out of the ground. Just like a college holiday in reality, every child's mother goes to school to pick up her children.
The poem also mentions that "their home is in the sky, where the stars live", and their rain mother lives there. The last paragraph, "I can naturally guess who they are raising their arms to," is also written in this way. The rain mother came, and the flowers came out of the ground, raised their arms to the rain mother, entered her arms, and enjoyed her love and nourishment.
Flower's teacher is the sun. The poem mentions that "if they want to come out to play before school, their teacher will punish them for standing in the corner", which can be understood as that if the flowers break through the door (soil) before school, the teacher (the sun) will punish them. Needless to say, the positive effect of the sun on flowers is natural.