It's surely not difficult that I'm asking a question, whether if I will be in trouble with the authorities if I drink on the flight back to Canada?
Not so difficult, surely?
Actually yes - this forum's about Poland rather than Canada. But common sense should prevail, and if you're drunk and disorderly Canadian law will apply. On LOT, you can drink, and in Poland you can drink. Remember three things. Firstly, if you are drunk when you arrive at the airport they might not allow you on the plane. Secondly, if you keep summoning the flight attendant wanting drinks, he/she may decide not to give you many, especially since you are a teenager. Thirdly, never, ever argue with a flight attendant, especially about alcohol - you might not like the result.
Perhaps you shoyuld enjoy a few drinks in Poland (in moderation - teenage binge drinking is viewed as negatively there as anywhere else) and be sensible on the flight. It's a long journey back to Canada, the other passengers don't want to be sat next to a drunken 18 year old and you don't want to feel like shite when you get there.