My understanding: The school of flowers is the soil, the mother of the flowers is the rain, and the teacher of the flowers is the sun.
In the poem, it is written that "flowers go to school in the underground school" and "they close the door to do their homework" both express that the soil is a school.
The mother of flowers is rainwater, and the content expressed in the poem is also very obvious: "When the rain comes, they take a vacation." It means that when the mother of rain comes, the flowers burst out of the ground. Just like when schools are on holiday in reality, every child’s mother goes to school to pick up her children.
It is also mentioned later in the poem that "their home is in the sky, where the stars live", and their rain mother lives in that place. The last paragraph "I can naturally guess who they are raising their arms to" is also written, Mother Rainwater is here, the flowers broke out of the ground and raised their arms to Mother Rainwater, entered the arms of Mother Rainwater, and enjoyed Mother Rain’s caress and nourishment.
Hua's teacher is the sun. The poem mentions that "if they want to come out to play before school, their teacher will punish them by standing in the corner." This can be understood as if Yu Yu's mother did not come to pick her up before school. If they, the flowers, break out of the door (soil), the teacher (sun) will punish them. Needless to say, the positive effect of the sun on flowers goes without saying.