What is a healthy weight loss goal for me?

My friend and I have been wanting to lose weight so we've made sort of a competition about it (we're both quite competitive and thought it a great opportunity). We're setting goals for ourselves, and I need to know what's reasonable. I'm 5'7, 14 years old, female, and 175 lbs. In 10 weeks, does 155 sound like a healthy goal or is it too steep? I want to get healthy without harming my body.