Basic biology: Girls need to know how their vagina works and how the parts of the vulva are named. Goes really for girls from all backgrounds - too many women don't seem to be aware that a vagina needs to be acidic and should therefore not be washed with soap, that antibiotics adversely affect the bacteria there, etc.
Facts on how to deal with menstruations would also be useful - schools usually teach you why you menstruate but aren't terribly useful when it comes to practical matters.
(Also, give info on where to get more information.)
Sex: Immigrant girls will face some contradicting messages. I'd focus on equipping them with all they need to avoid being coerced into having sex. Don't put on rose-tinted glasses when it comes to how that works; "just say no" isn't going to cut it. Some males are rapists, and a girl who grew up in a culture even more patriarchal than ours will be ill equipped to fight back against a male.
So don't teach them that it is all okay to be alone with boys when their culture of origin says otherwise, and the rate of date rape kinda also does.
Teach them to think about what they want, and then about how they can best achieve that. Teach them that it is fine to tell a boy "my parents wouldn't allow that" if he doesn't leave them alone. Teach them how to go to the toilet and vanish through the back door if a man molests them in a bar. In short, prepare them to deal with the very real risk of a "no" being ignored.
Emphasize that it is okay to be different from their peers in what they will do and won't do.
I've met some (highly educated!) Muslim girls at uni who seriously thought unwanted pregnancy would never happen to someone from their cultural background.
So, if you have that kind of people, why not go along with their beliefs and give them the contraception and consent lesson in the context of marriage? Be honest about the real state of affairs, i.e. that marital rape is officially illegal, but it is really hard to get a rapist punished if he was married to the victim.
I feel that girls from cultures that are against divorce and against sex before marriage deserve to know exactly what they get into when marrying, considering that marital rape is not considered a crime by those subcultures. (That includes Christian extremists)
Go through the whole range of contraception and emergency contraception, perhaps with a bit more of a focus on practical matters, as unsupportive parents might be a higher risk.
When it comes to boys; I actually don't believe that boys and men are at all confused about consent. Don't have the link here, but there's even studies on the matter; and men are proven to be very well able to recognize a "soft no". They just often choose to ignore it when it is inconvenient for them.
So instead of teaching boys that no means no, I'd focus more on trying to teach them empathy for girls. "What do you think, how does a girl feel if ...", that sort of thing.
Advice on condoms is good. Advice that they don't have to have sex may also be needed, depending on how much boys are pressured by their male peers to have "conquests".
And maybe a "throwaway" comment about how "real men" want to make sure their partner is actually having fun and being pleasures, not just pretending to be.
Um, no. Masculinity is toxic. It being a matter of vanity for men to get a woman to orgasm has changed things from women being allowed to be honest about their lack of enjoyment to having to pretend orgasms.
Teach boys female anatomy so they CAN please a woman in bed, and teach them empathy so they WANT to.
Making it a matter of male vanity would only make things worse.