It could be one of two things that make the guys say this about you . It could be the way you present yourself to them . You may be sending signals that you're not meaning to send . Certain actions you take, or clothes you wear, how you do your makeup, or even your attitude . Remember that 95% of the time, guys are thinking about sex so anything that excites those thoughts can be considered easy .
The second reason could be that maybe they WISH you were easy so they could have a chance with you . You know how they say if you hear something enough you'll grow to believe it ? Maybe they think that by telling you that you come off that way, you'll eventually be like well I hear this enough, I might as well do it . That's a possibility but I've had mostly guy friends my whole life and have never had them say anything like to me, so you may want to analyze the first possibility .
And especially in high school, the majority of girl friends are sooooo fake ! They could be thinking that you come off as easy but because they're your "friends", they wouldn't dare tell you to your face because it's so much better to tell you what you'd want to hear and tell everyone behind your back how it really is .