Wellington College Principal Roger Moses was notified by a member of the public.
He said the nature of the comments were "appalling", and confirmed the school was investigating.
"I'm not going to try to defend what was said one iota.
"There's some pretty horrendous comments made, in what they probably saw as the privacy of a closed Facebook group.
"The reality is, there's a learning curve here for young men who think they can say things with a sense of bravado, but these days anything is discoverable."
Moses said the school would "deal with" the boys, but that the students who made the comments were "shattered" by what had happened.
The students involved were senior pupils at the school.
"We're trying to draw a distinction between comments that we absolutely abhor, and a couple of students who are normally good lads," Moses said.
"We'll launch an investigation into what's happened, and we'll deal with it in time.
"But we're not sure exactly what is going to happen because we need to know the facts of it."
Moses said that young people often had technological knowledge that outstripped their moral development, and they ended up saying outrageous things "without seeing the implications".
He said that was an issue being faced by everyone working with young people.
Wellington College was currently trying to trace whether the comments had been posted during school hours.
New Zealand Association of Counsellors spokesperson Sarah Maindonald said the comments were shocking.
"I mean, what they're talking about is rape, really.
"We've got to take a stand on this as a society.
"Suggesting that, to be a Wellington College boy you have to sleep with a girl who's drunk, that's just the ultimate objectification of a girl for their gratification.
"That's really worrying."
Maindonald said the principal was in a difficult position, but his comments weren't helpful.
"Because they're usually 'good lads', we can excuse this behaviour?
"That's a really mixed message, you know, that 'lads make a mistake'.
"I think a lot of messages boys get around fighting, violence, and sex, are quite old school.
"But we've got to give young men and women the skills for proper relationships."
Maindonald said more counsellors were needed in schools, to ensure students got proper guidance.
She said there was currently very little resourcing for programmes that helped young people develop interpersonal skills, and understand topics like consent.
Wellington Sexual Abuse HELP general manager Conor Twyford said the posts were worrying because of the "complete lack of consent".
"People are starting to talk about 'enthusiastic consent'.
"So it's not enough to say someone didn't say no, or they gave consent haltingly, or they were inebriated and not in control of themselves."
She said young people needed help to understand what consent and good relationships looked like, but at the moment it was under-resourced.
"Only 1.5% of the whole government spend is on prevention," Twyford said.
"This is a wider cultural problem for New Zealand, and young people are at the forefront of a cuture change.
"Stories like this show us that there's so much more work to do."
