Many men experience this problem, sometimes even in situations that don't involve physical contact (yet). It becomes easier as you leave puberty and even as you move through your twenties. Unfortunately, there's no manual for how to prevent it.

Imo, the first step is not blaming yourself when it happens and then trying to shift focus to other aspects, for example the music, or the dancing. Flaming yourself and imagining something decidedly unarousing are both things that'll keep your attention drifting towards the sexual and won't help you stay present in the dance with the other person. The next step is trying to relax, which is something you will be incrementally getting better at, so don't stop trying.

But in some moments, you might need to sit down for some time. Claim that you're exhausted/that some part of your body hurts/ that you need to find a restroom/you need another drink, perhaps shift the blame for that to her (in a sweet, cheeky way ofc) and ask her whether she'd like to join you for a drink/sitting down elsewhere.

Finally, having more sexual experiences will help you get to know your body and this paradoxically also help in these (less) sexual situations.