If you can swing it, see if you can get a suit that is fitted. You don't have to go to great lengths, but most good places will make some adjustments to a suit that is almost right but needs a few nips and tucks. If you were grown, I'd say to get one that you could keep for other occasions, but if you are still growing, you are better off in the mid range.
Man, I wish I could have worn a suit to our 8th grade dinner dance. My folks made me go and wear a dress. Being completely clueless on female dress, I didn't know you had to wear a slip. After the dance, I was standing outside with everyone else waiting for rides and in the headlights of several cars. One girl came up to me and told me to move out of the light because they could all see through my dress. Funny now, fall through the floor embarrassing then.
Get something you are comfortable in. Burlington used to do some adjusting if you bought a suit. They may still do that for you, if not free than for a modest price.
sam1234