How are we supposed to get promoted if ALC classes are filled?

Try getting on the wait list. I know for my mos it's hard with the new step program as well. However, you have a lot of people on the OML that don't meet the standards when their slot comes up I.e. Overweight, failing apft, etc. you can also try attending a national guard or mtt. They have open slots that your training NCO can try to reserve as well. It's tough but you got to stay motivated and exhaust all options. Don't give up as you are already ahead of the game trying to go before being promoted. Good luck to you

Ahh the joys of attrs controlling your advancement

Wow that's a very tough question and I can certainly understand your frustration. I can only suggest you get a hold of your branch and continue to push the issue of trying to get in the first available school.