Just separate yourself from her as much as possible.
By the way, I dated seniors when I was a freshman. My last boyfriend was a senior, he graduated in 2011 and I graduate in 2014. Same case for the guy I currently like and am interested in. He's 19 and I'm 16 and he'll turn 20 right before I turn 17. He graduated in '11. I don't see the big deal. A girl can easily have sex with someone her age just as easily as someone older. I don't think it's really true that ALL guys just sit around and play video games, even so, older guys do the same thing. That is your opinion and I respect it although it was wrong of your step-mother to attack you about your opinion.