How can i get my money or weed back from a kid who is bigger than me?

So on Halloween night i wanted to buy some weed. My usual hook ups were all gone partying and i couldnt get it from them. i was getting candy in my neighbor hood at the time because i didnt have a ride any ware. So this kid that goes to my school (junior) said he could get me some weed. so i gave him the money and he said he would meet me right where we were in an hour. 2 and a half hours later he didnt show up. I was pretty ****** pissed. So the next day at school i asked him where my weed was. and he didnt have it. so i asked for my money back an he said it was gone. then i said WHAT THE **** then he walked away. This kid is a junior and im only a freshman. I can fight an im pretty buff but this kid is fat as **** and way taller. what should i do? I was thinkin about bringin a small knife and threaten him. But i dont want to get in trouble. any suggestions??