@bubu,
Dear sir,
This letter is to bring
to your kind
attention [notice] [to] the reason I failed to write my lesson plan for 23rd June and
to explain [justify] that it was only a mistake and not deliberate negligence.
Sir, on 23rd June, just before I left the school, I received a call on a very important and pressing family matter and I had to
leave quickly [rush off in a panic]. Unfortunately, I had left the lesson plan register in the school office cabinet. Therefore, when you came the day after, you did not find the lesson plan done [in it].
I know it was my mistake to forget the lesson plan register at the office and I apologize profusely.
Sir, you will already have seen that my lesson plans were complete up to 22nd of June. I need to bring to your kind consideration that it was my first mistake and pray to be forgiven this time. I swear that this mistake shall not be repeated.
Yours faithfully,