as explained above, it really just comes down to having sex during a brief fertile time period...it doesn't make a difference how many times it's been done, just when it happens.
you have to remember, in high school they just want to get across the message to not have sex, or else. Technically, it's just not biologically true that it can 'happen any time'. It can only happen a few days out of every month. Of course, most HS girls have no clue when they ovulate, so it's easier to just tell girls not to at all.
as for due dates, they aren't perfect. they are close, but it's reasonable to be a week different.