Dear Sir/Madam/你想要致歉的人
I am terribly sorry that I wasn't able to clear the blackboard well due to a lack of good responsibility.This insidence made me realize that I caused inconvenience to the teacher who was comging to use the board.I would like to ensure you that this would not happen again in the future.
Yours faithfully,
(你的名字)
另,可怜的孩子,教师节刚过,哪个老师对你这么凶?让我来和他/她谈谈.