I've been to countless bars and izakayas in the company of people not 20 and no one was carded. The same people regularly bought booze from conbinis and never got carded. I've had several minor friends who were so drunk in public the cops actually did stop them and write down their gaijin card information. Nothing ever happened.
I'm not saying no one ever gets in trouble, but it's really not the super crack down you think it is.

The OP was asking about getting into clubs, right? I am not sure if that means he wants to go clubbing, or wants to go drinking.

I have another question.

What forms of ID does Tokyo clubs accept? Passport, UK driving licence?? or will I have to have some other form of ID?

Passport or Alien Registration Card. Don't count on foreign license being accepted.

Do school IDs in Japan have birthdates?

Definitely not "going drinking" ;)

Does anyone here know if Japan does alcohol-free-zone club nights for minors?

Yeah, does anyone know?

I don't think so, as alcohol isn't demonized as it is in the West. Are teens in japan into dancing/clubbing? I remember the clubs of the 80s and 90s, but I think most of those are gone now.