I had a similar thing happen in high school, which I've always remembered.
I was about 250 pounds all through high school, although I played volleyball, basketball and did track & field. So while I was pretty strong and fittish, I was by no means classicly good looking - I was pretty chunky. I never dated, just had girlfriends, and was very self-conscious about my height (6 feet by grade 10).
Then one day in English class in grade 11, I was passing papers back to the person behind me, who was this boy I didn't really know (part of the smoker/partier crowd) - not at all part of the group I hung out with. I didn't know him at all, really. He asked me something, so I answered it, and while I did he looked for a bit longer than normal at my face, kinda staring. Then out of the blue, he says 'you're really beautiful, you know'. I blushed from my toes to the roots of my hair, had no idea what to say so I just spun back around and said nothing.
I'll always remember it though. Those little moments of kindness that really lift the spirit.
So glad you had one too!