Endurance running is the best exercise to reduce fats from thighs, it will also keep your body in shape and make your body strong.
A healthy diet is also very important for reducing weight.
1: walking, jogging will help tremendously
2: Swimming
3: Squats
4: Skipping
last but not the least try zumba, aerobics
First question would be, what do the adult members of your family look like? That will establish your genetic predisposition. As for exercises, you can either tone your thighs, which might not make them any smaller but will make them more muscular, which would involve resistance exercises. To just get smaller, aerobics would be your best bet. But how's your diet? Eating too many simple carbs such as sugar or white flour can leave fat deposits because if not quickly burned it will store as fat. Also, are your thighs fat, or just large? Again, looking at the adult members of your family, especially of the same sex, will give you an idea of your normal body shape. As for specific exercises, if you have access to a gym a personal trainer can help. At school, you can look into aerobic sports such as soccer or swimming or running, which will combine fun with cardio. But keep in mind, you will also add muscle. Take a look at Serena Williams -- she's got large but powerful thighs which have made her almost impossible for her competitors in pro tennis to beat. But she'll never have small thighs, it's how she's built. She just put them to work for her.