Yes it is against the law. Consensual sexual activity between a 15 year old and an adult is called Lewd and Lascivious Battery and is a sex crime, meaning a conviction results in the person being a convicted sex offender. It is punishable by a maximum of 15 years in prison.
Interestingly, if the boy turns 16 and the girl is 19, then it is legal for them to have sex. A person 18 years or older but UNDER the age of 24 can have sex with a 16 or 17 year old.
This is not to be considered legal advice nor does an attorney-client relationship exist.
Mr. Trabin's answer is correct. The crime being committed is a second degree felony. It's also stupid.
This answer was provided as a courtesy to you and no attempt was made to establish any type of attorney/client relationship.
Yes, consensual and in high school are not defenses in this situation.
Please be advised that nothing in this posting is meant to supplement the advice of an attorney and The Law Office of Robert J. Lee, P.A. and/or Robert J. Lee has not undertaken any professional responsibility with regard to your case unless otherwise agreed. RJL is licensed to practice law in FL and MA, but no other jurisdictions. Nothing stated herein should be implied as legal advice.