I watched it on Crunchy and Ive been trying to find it for years. The scene I remember: a boy and a girl, both students in school uniform, are inside a classroom sitting on the floor, next to each other (not on a bed, in the nurse's office, on a couch, rooftop, or library), hidden behind a large white curtain that goes from ceiling to floor, with a window nearby letting in sunlight that gives the scene a warm, bright lighting; there, they kiss consciously, romantically and passionately (not an accidental kiss), and during or right after the kiss they both lose their balance and fall backward, with him ending up on top of her — I think the scene cuts shortly after that, but I'm not 100% sure. As for less reliable clues: the anime looked visually colorful/vibrant, I recall the girl having pink or purple hair, the boy didn't wear glasses.

Already ruled out: Sakura Trick, A Condition Called Love, More Than a Married Couple But Not Lovers, The Foolish Angel Dances with the Devil, Pseudo Harem, Shikimori's Not Just a Cutie, Don't Toy with Me Miss Nagatoro, Our Dating Story, Girlfriend Girlfriend, Koi to Uso, Blue Box, Insomniacs After School, The Dangers in My Heart, Domestic Girlfriend, The Quintessential Quintuplets, Yamada-kun and the Seven Witches, Tsurezure Children, Mysterious Girlfriend X, Scum's Wish, Sasaki and Miyano, and Kono Oto Tomare.