Both are fine. You don't have to, but it's nice to do so.
Teacher, are these sentences OK? Do I need to say "I am sorry"?
I am sorry I will not be able to come to the department lunch as I am off that day.
I am sorry I will not be able to attend the department lunch as I am off that day.
Both are fine. You don't have to, but it's nice to do so.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.