I will write some minor comments.
僕はもう12年生で、would be僕はいま12年生で because もう includes the meaning of already surprisingly.
I don't know that the 補習校は10月19日まで休ませて頂きますので、could be the reason for your absent. For example, adding 実家に帰っていて at the beginning would be better for clarification.
Anyway, the sentences is really good level. 😊