You've described two different crimes - forcible rape and statutory rape. The definitions of rape vary from state to state, but here is the definition from my state of Washington.
link states that rape in the first degree is when a person engages in "sexual intercourse" (which as you'll see by their
link is any penetration, "however slight") by "forcible compulsion" (defined as "physical force which overcomes resistance, or a threat, express or implied, that places a person in fear of death or physical injury to herself or himself or another person, or in fear that she or he or another person will be kidnapped.") In other words -
NO MEANS NO. If the victim is non-compliant, and does not consent, it is
rape. Keep in mind that this definition applies to male victims as well.
The
link are even more specific. They define first degree sexual assault as "a person who subjects another person to sexual penetration, or who forces another person to make a sexual penetration on himself or herself or another, or on a beast, against the will of the victim or under conditions in which the perpetrator knows or should know that the victim is mentally or physically incapable of resisting or understanding the nature of his or her conduct, is guilty of sexual assault."
So like I said, those are the legal definition of rape, but as you can see, it all comes down to
consent. If the victim does not consent, then it is rape. If the victim says "stop" or "no" to sex, even if the victim initially seemed willing, sexual activity should
stop immediately.
The scenario you described is another crime entirely, called
statutory rape. A person is in commission of this crime if he or she has sex with a person under the age of 18 (or the age of consent in the state/country it's committed). Stat rape is a crime regardless of whether or not the minor consents to it, because the law does not recognize minors as being capable of making informed decisions, and therefore are incapable of consenting. This is also why minors need parents or guardians to consent to other serious decisions including tattoos, piercings, marriage, and surgery.
Even if it's not stat rape, rape between a teacher and a student (maybe an 18-year-old) is still generally against the code of conducts of most schools, on account of the teacher is in a position of power over the student, and having sex with students is seen as an abuse of that power.
