You can delay it indefinitely, as you’re now at the interview stage it’s likely to just be scheduled, but you can cancel it. You could also try contacting NVC.
But as somebody that moved from the UK with two teens, one of whom had just done his GCSE’s, then I’d say unless you’re putting them in to a British school then don’t delay. GCSE’s are really no use to them either side of the Atlantic, and it would be better to get them in the US school system asap to give them more time to adjust. Mine went to a British school so it was a little different, but if he hadn’t been then waiting for him to complete GCSE’s would have been pretty pointless. I know you didn’t ask for advice but honestly I can’t see any reason to wait a year and think it would be more detrimental than helpful. Particularly if they think they might like to go to college in the US.
Best of luck.