I believe it was high school based. Male MC is a nerd/geek type and has a van. Female MC who is in the popular cloud and usually would not think of being with a nerd, takes a dare to get male MC's underwear(I think), or go on a date with him.
After rocking the van a couple of times the male says he learnt about making love with his sisters. I think they stay together afterwards.
Not much to go on, possible not on this site.
It's an old Lubrican story, from back when he called himself Beating off Bob. I think its one of the "blues" stories, maybe "cheerleader blues".
The Cheerleader Blues by Lubrican (premier members only)
https://storiesonline.net/s/44878/the-cheerleader-blues
https://storiesonline.net/a/Lubrican
That is the one. The first chapter matches perfectly. Good thing I got to read it when it was free(either on here or the 'as' site). Thank you.