Final answer:
August and his mom leave the school abruptly for various reasons, including catching a boat, avoiding farewell scenes, and permanently leaving.
Step-by-step explanation:
August and his mom leave the school abruptly in the given text snippets due to various reasons. In the first snippet, Trina mentions leaving early in order to catch the last boat, which suggests that they have to leave the area where the school is located. In the second snippet, Kismine mentions that they avoid any farewell scene, implying that they leave abruptly to avoid emotional goodbyes. In the fourth snippet, Trina and her family are seen leaving, and Trina asks her mother to write to her about August and the twins, suggesting that they are permanently leaving the school and will not see August and the twins for a while.