She will have a baby. You will be stuck paying child support for this child until it reaches the age of 18 or 21 or possibly 23. You will likely have a hard time going to school to get a better job because you will be working to pay for the baby's needs. The stress on your relationship with your GF and her folks not to mention your folks will be very strained for at least a while and you and she may not survive it as a couple. Is it worth it for a few moments of fun??
Learn to use contraceptives (rubbers etc) if you are going to be sexually active. Take responsibility for your actions now before you have to be responsible for a little innocent life.
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Criminally -- probably nothing. It isn't statutory rape since you're the same age, and its not forced. But I agree with the other poster, getting a girlfriend pregnant at 16 will be a life changing event for both you so either abstain or be responsible and use protection.