In chapter 1* of On Writing, King says that Carrie White was based on two girls that he knew from high school. All the details mentioned in the question are there, but not in those exact words.
It is likely that Wikipedia's source for their quote is Secret Windows.
muru has left a comment with a link to a website with the exact quote, attributed to an interview of King in the April 1981 issue of the Twilight Zone magazine.
And I also helped myself, digging back to my memories of high school
(my job teaching English didn’t help; I was twenty-six by then, and on
the wrong side of the desk), remembering what I knew about the two
loneliest, most reviled girls in my class—how they looked, how they
acted, how they were treated. Very rarely in my career have I explored
more distasteful territory.
I’ll call one of these girls Sondra. She and her mother lived in a trailer home not too far from me, with their dog, Cheddar Cheese.
The other girl I’ll call Dodie Franklin, only the other girls called
her Dodo or Doodoo. Her parents were interested in only one thing, and
that was entering contests.
Whatever the Franklins might have won, a supply of clothes for growing
teenagers wasn’t part of the haul. Dodie and her brother Bill wore the
same stuff every day for the first year and a half of high school:
black pants and a short-sleeved checked sport shirt for him, a long
black skirt, gray knee-socks, and a sleeveless white blouse for her.
Some of my readers may not believe I am being literal when I say
every day, but those who grew up in country towns during the fifties and sixties will know that I am.
Dodie and Bill Franklin got on all right at Durham Elementary, but
high school meant a much bigger town, and for children like Dodie and
Bill, Lisbon Falls meant ridicule and ruin. We watched in amusement
and horror as Bill’s sport shirt faded and began to unravel from the
short sleeves up. He replaced a missing button with a paperclip. Tape,
carefully colored black with a crayon to match his pants, appeared
over a rip behind one knee. Dodie’s sleeveless white blouse began to
grow yellow with wear, age, and accumulated sweat-stains. As it grew
thinner, the straps of her bra showed through more and more clearly.
The other girls made fun of her, at first behind her back and then to
her face. Teasing became taunting. The boys weren’t a part of it; we
had Bill to take care of (yes, I helped—not a whole lot, but I was
there). Dodie had it worse, I think. The girls didn’t just laugh at
Dodie; they hated her, too. Dodie was everything they were afraid of.
After Christmas vacation of our sophomore year, Dodie came back to
school resplendent. The dowdy old black skirt had been replaced by a
cranberry-colored one that stopped at her knees instead of halfway
down her shins. The tatty knee-socks had been replaced by nylon
stockings, which looked pretty good because she had finally shaved the
luxuriant mat of black hair off her legs. The ancient sleeveless
blouse had given way to a soft wool sweater. She’d even had a
permanent. Dodie was a girl transformed, and you could see by her face
that she knew it. I have no idea if she saved for those new clothes,
if they were given to her for Christmas by her parents, or if she went
through a hell of begging that finally bore dividends. It doesn’t
matter, because mere clothes changed nothing. The teasing that day was
worse than ever. Her peers had no intention of letting her out of the
box they’d put her in; she was punished for even trying to break free.
I had several classes with her, and was able to observe Dodie’s
ruination at first hand. I saw her smile fade, saw the light in her
eyes first dim and then go out. By the end of the day she was the girl
she’d been before Christmas vacation—a dough-faced and freckle-cheeked
wraith, scurrying through the halls with her eyes down and her books
clasped to her chest.
Italics are in the original; bold is mine.
As an interesting aside, King continues:
Both Sondra and Dodie were dead by the time I started writing
Carrie. [...] Dodie married a TV weatherman who gained something of a reputation in New England for his drawling downeast delivery.
Following the birth of a child—I think it was their second—Dodie went
into the cellar and put a .22 bullet in her abdomen. It was a lucky
shot (or unlucky, depending on your point of view, I guess), hitting
the portal vein and killing her. In town they said it was postpartum
depression, how sad. Myself, I suspected high school hangover might
have had something to do with it.
I never liked Carrie, that female version of Eric Harris and Dylan
Klebold, but through Sondra and Dodie I came at last to understand her
a little. I pitied her and I pitied her classmates as well, because I
had been one of them once upon a time.
*I have the e-book, so my page numbers are not going to help you much. I'm not really sure how the book is arranged, but it appears that this passage is in section (?) 29.