morally I dont know the situation good enough, but I generally go by this, if she is withing 3 years, like if she turns 15 before I turn 18 then its ok, but that is pushing it alot.
Also just casue your 17 doesnt mean its legal, in florida the age of consent is 16, so a 16 y/o with a 15 y/o is illegal, but probly wouldnt get much punishment if any in court, but in your case 17 with 13 that looks very bad that your talking advantage of her being naive or something whether she is or not. Go on google or yahoo and look up consent rules in my opinion and then make your decision, but if she is in high school morally I think it would be fine.