At 17, you're also still growing!
At 5'6" or 5'5", you're at the lower end of your healthy BMI at 126 - I'm 5'4.5", and 110 is at the lowest extreme end for me before I am underweight, for example. It might be that your body is just at a nice ideal weight. How do you feel about your body? Is the number driving you regardless of how you look or feel? Numbers are less important than that, in my opinion. I'm not trying to dissuade you, just looking at it from another angle! It sounds like you're already pretty fit!
I agree with handcycle - if you have the ability, try to figure out your lean body mass (fat percentage). It should be around 20-25% for the average fit woman (if you're an athlete, being as low as 15% is okay). If you're a guy, around 15% is considered a good number for fitness (maybe slightly lower) and if you're an athlete, around 10% is fine. Women in general should have more body fat
You could also try incorporating more strengthening into your workouts - muscles will help burn more calories at rest. Are you drinking lots of water, as well?
Female, 31 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 10/22/14 = 175.0 (net: -40.2)
Last mini-goal: 175 lbs. REACHED 10/22/14 --REWARD: New wallet
Current min-goal: 170 lbs.
Next large goal: 165 lbs. by 12/25/2014
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130-140 lbs.
"You don't have to change your life today. You only need to change your day today."