No, it's not most. It's 1 in 3. Around 20% of pregnancies end in miscarriage.
I think the figures make sense. Most women will have more than one pregnancy in their lifetime, so more than a 1 in 5 chance of experiencing miscarriage, but some women won't have any pregnancies at all, putting them in the 2/3, and some will only ever have one or two pregnancies resulting in children, because they just happened to get good odds and not have underlying issues which cause miscarriage. The later you get pregnant the more chance you have of miscarriage, and of course the more times you get pregnant the more chance you have of miscarriage overall, so women with three or more children are more likely to have experienced miscarriage - but then, some women decide after a miscarriage not to try again, or might have issues conceiving so have a smaller number of children not by choice.
I have had two at five weeks which is "one week" as you put it, I saw a doctor and was scanned and a pregnancy was confirmed even though it wasn't successful. My "period" would have been two weeks late. Yes, it is a miscarriage. It's referred to as such in my medical notes. Chemical pregnancy refers to a pregnancy which is lost so early that it's not possible to see any evidence on a scan etc so the period starts as usual or a few days late.
Noni's biology teacher's comment is baffling. I expect she meant eggs which are fertilised but never implant, but this isn't a miscarriage or a chemical pregnancy, because not implanting means no hCG is produced which means no positive test or pregnancy at all, medically speaking. But even this isn't 90%, it's more like 60%.
DH has a genetic issue which causes ours. Our miscarriage risk is much higher than the standard 20%. Before I met him I had DS without any miscarriages before him, and none of the other women in my family have had miscarriages. I definitely know people who have had miscarriages, but not all of my friends with children. Of course there are a few I know haven't, and a few I know have, and some others that I don't know because the subject has never come up or they've never explicitly said so.