Can I be excused?
A request to leave something in particular, often the dinner table. OK, I ate all of my vegetables—can I be excused now?
Could I be excused?
Would you give me permission to leave?; Would you give me permission to leave the table? (Also used with can or may in place of could.) Bill: I'm finished, Mom. Could I be excused? Mother: Yes, of course, when you use good manners like that.