Hi Soapy, what a horrible situation to be caught in, and so disruptive for your studies. You know you can't help him, as he has to want to help himself and his insensitive joking would just make it worse.
Think about what support you can access. There is Alateen, school counsellors, maybe extended family like grandparents, aunts or uncles. School especially may be able to put you in touch with other support services. It's important you stay safe, so if you have any concerns like him driving you drunk or smoking and falling asleep, I suggest you tell your school about it right away.
Secrecy doesn't serve anyone but the alcoholic, so don't feel you have to hide your Dad's condition from relatives and friends. They need to know so they can help you.