wow, that is horrible advice to tell someone no matter what age you are!!!
Do not go out and drink and think the world is horrible and life sucks. That is not the case, you life is what you will make of it and how you view it. If you want your life to be great then you make it great and that is how you have to live. Finish school, go on to college and make something out of yourself and that is the best thing you can do for yourself. If things continue I would HIGHLY suggest listening to the idea of being inventive on hiding your money, stand up for your self and tell them NO. Do not give in and do not give them anymore money, tell them you worked for it, you earned it, it is YOURS to keep. If they have monetary issues then stealing money from a 15 year old or even asking them is not how they should be solving their money problems, you can't just simply rely on your kids as if its ok... maybe they need to consider things they can cut back on ect. As said before you can talk to them about it... and ask them if they need help, and say you will pay a certian bill or something (that you can afford and still have left over money) but thats as much as I would offer after they stole money from you (thats being nice, if it were me I wouldnt give them anything ever).
IMO I would seriously find a SUPER good hiding spot for your money, save up as much as you can (def. make sure they can't find it) and if you keep saving by the time your 18 you make sure you keep your job, work enough hours and look into moving out, rent a place with a friend so its cheap. The more you save the better off you will be, but remember to hide it good, if they find it thats a lot of money to lose... which I would think you have probably lost a lot as it is. I just think that is extremely wrong of a parent to do to their kid.