Wouldn't recommend it. You could get in pretty serious trouble for trying to scam them when they find out, and they will find out.
Assuming you're talking about yourself and it isn't a hypothetical question, and that you want to try this because you don't think the sixth form will take you with your actual grade:
Try asking if they'll take you if you resit during y12, I knew people who did that at my sixth form with Maths and English.