When my kids were in highschool and their friends would ask to borrow money they would loan them a couple of bucks. When they would ask again for more money, my kids would remember and say "when you pay me back the money you already owe me". That was the end of that.
I'm not sure how good of friends are asking, and their age, and how much they want. To me those answers would determine what I do. BUT you could just have a policy of "no" to anyone and everyone. It's your choice.