I am 16 years old, female, 5'4" and weigh 143 lbs. Am I overweight? And is it possible for me to lose 15-20 lbs in one month?

Normal weight. At 24.5, your body mass index ( BMI) is in the normal range. Losing more than 2 pounds per week is not healthy.
Overweight. You are not overweight Your BMI is 24.54, normal is 18.5 -24.9. Losing 15 -20 lbs a month is a lot.For weight loss cut back on 500 calories a day which is about a pound a week, (3500cal is 1- lb) so 15 lbs a month is quite drastic. Since you are normal weight , if you just wanted to trim a bit cut 500 calories a day for a month. Consider discussing body issues with doc, f/u w/doc.
Anything is possible. But it's not ideal to consider such an aggressive weight loss regimen. To lose weight at a pace like you mention, may place you at significant health risks. If weight loss if your goal, consider a more reasonable goal of 1-2lbs/week. Since your a minor at 16, if you're planning to embark on a weight loss regimen, you should review this with your parents.