for mock results its unlikely you'd get caught, unless you get a school report sent home or your parents see your teachers at all in which case it's likely that your mock result would be mentioned, or of course you slip up and are inconsistent with your lies
your real results it depends how your school give them to you, at my school we could collect our results, but then a letter was also sent home with the results, there is always a risk of them seeing that letter, or seeing the results you pick up
probably just best to be honest with them, they'd probably rather know you didnt do as well as you hoped (assuming you didnt otherwise why lie) than find out that you lied to them and you didnt do as well as you had hoped