I advise you to stop these relations. Not only are they bad for your job, they are bad for you as a person.
Starting a relationship out with just sex isn't starting out a relationship well, if at all. She's using you.... I agree with pheasant in that your dad isn't giving you good advice. Anyone can have sex, but it takes someone special to have a good relationship and to actually date and you don't want to make yourself cheap.
I know you think you are a "consenting" person, but it's true that it may not be legal. It is different in various states. The reason why there are laws against this is because the minor party (yourself) doesn't fully realize what they are getting into. That law is there to protect you. What you do need to know is that basically you are being taken advantage of and nothing more. Don't make yourself such an easy target. It's hard at 16 to understand how people can be that way, but that's the way it is....take it from someone who has been there.
Since you are only having sex, then there is no sense of commitment on her part, and there is a chance she could drop you for someone else in the blink of an eye, and she most likely won't want to develop it further.
I had more things I wanted to say, but I have so much bring in my mind that I can't type it all at once. I hope this is enough for you to realize the harmful situation you are in, and I hope you make the right decisions.