You could always laugh and say "are we comparing?"
or put on your serious face and say "If you're concerned X isn't progressing, then it's better to speak to the teacher and they'll be able to help, rather than try to compare with other children in the class."
or you could say "Why are you so interested in what level my X is on?"
Or you could just tell them, on the grounds that it doesn't actually matter if they have that information
Or you could say "really, that's none of your business and I find it perplexing how intent you are on finding that out, why are you so desperate to know?"
Or you could find out what the very top level is and say that.
Or you could put on your concerned face and say "you ask me that question all the time, is everything ok?"
or you could say "oh, I don't do that stupid school gate competitive stuff" and laugh.
there's loads of stuff you could do.
Of course, it's best to first make sure you know that that they aren't asking because they are actually worried about their child.