South Park Head Canons
~With how fucky wucky South Park ages and time passing is I would like to say in my HCs Shelley is only Two grades ahead of the boys and the age gap with cartman being around the year - year and a half range~
Development Of SheMan
(Or shartman hehe)
After Cat Orgy we do get to see small instance of Shelley and Cartman interacting. And we know from the few times they team up they are an absolute unit to be dealt with.
Personally I don't Shelley gets 'Pretty' until the summer between 8th and 9th. When she gets her headgear off. Puberty has a bit more time to grow into her features. And of course her getting time to grow more confident in herself.
So I think the first instances of Sheman inclinings start at Shelley's 8th grade formal.
Being a common guest at the Marsh residence. Cartman finds Shelley crying to herself about no one wanting to go with her. And like we've seen before Cartman helps pick her up by her boot straps and cheers her up. Promising to go with her.
The other kids make fun of Shelley for hanging out with a 6th grader, but she doesn't care and they have a great time together. Moving a mutual respect to a great and tortuous friendship for the rest of the guys.
By the time Shelley's senior prom rolls around she's become pretty popular. She's a starter for the Highschools wrestling team. Gotten an Athletics Scholarship for college. And is the go to person to get a DVD burned. While she definitely has the choice of the bunch of who she could go to prom with. She decides to go with her long time buddy Cartman.
Nothing activley happens between them that prom night but it definitely seals the deal for the small crush both had grown for each other.
By the time for Cartman and subsequently the rest of the guys prom rolls around. Shelley and Cartman are happily together. Much to the dismay of Stan. She comes down to visit and is his date like he was hers.