Sometimes children will say things like "I promise to do/listen/focus better" when they really want something but don't know how to do it. They know it's the "right" answer if that makes sense, (or have learned it's not a good idea to be honest and say "I can't" or "I don't know how") but it might be that he needs some more specific measurable goals. The instruction to "Focus and listen to instructions" is a bit vague even for a 12 year old.
Ask the teacher what specifically it is that he is doing - what form does this "not focusing" take? Is he talking when he should be listening, showing off to others, getting out distracting things like phone/toy/etc? Then be more specific in what you want him not to do and what you want him to do instead. Try to get him to be honest about whether he thinks he can do this and if he isn't sure then try to come up with some strategies. Little reminders of the strategies immediately before the class. Check in with the teacher after a couple of weeks and see if this is improved, make some adjustments if necessary, check in again after another 2 weeks. If no improvement after 4-6 weeks (whatever aligns best with the payment timing), maybe pull him out for now and try again when he's older, you could look for a different class which is "cool" - maybe a different kind of dance, or an instrument - drums? Or another "cool" sport: parkour, freestyle, skateboarding, martial arts? Is he into performing - magic tricks can be something fun to learn at home. Some might be more structured than others, and obviously will depend what's available local to you. Go back to street dance in a year or two, if he still wants to.
I also think the point another poster makes about dance being very disciplined and repetitive and boring is an excellent point. Sometimes a hobby can seem exciting when you see the final result, but it can be a real hard slog to get there. Most of the enjoyment of the hobby is the result of that hard work, but you don't get perfect overnight. And various hobbies completely differ as to where that point is - as a child/teenager I played various instruments but became frustrated and gave up after not very long with all of them because I just wanted to be able to play something that I liked straight away, I didn't want to spend weeks practising scales. Eventually as an adult I returned to acoustic guitar but instead of playing doggedly through a tutorial book I just taught myself to read tabs and a few chords and then started, immediately, playing simple versions of pop songs that I like. The work/result point came much earlier and I find it infinitely more rewarding now. OK - my playing isn't going to wow any audiences or win music awards but that doesn't matter to me. Being perfect isn't my goal, having fun is. Maybe it would be worth discussing it with him along these lines and seeing if it's really this hobby he wants to pursue or whether there are only certain aspects of it which he likes and he could look for those in other areas.