Reasons are explanations for actions. Eg. “The reason I was late turning in my homework is that I had to got to the hospital”.

Excuses are explanations for actions (aka reasons) that are not socially justifiable. Eg. “The reason I was late turning in my homework is that my friends invited me out for drinks”.

The hard part is that jackasses will view even valid reasons as excuses, because to them any factors negatively impacting you are not more important than the end result being achieved.