This doesn't have to be a HP crossover though I guess you could make it one, but somehow, the Scoobies wind up getting Sorted into the Hogwarts Houses by the Sorting Hat. Maybe Dawn, Tara, and Willow are geeking out about the books and Willow decides to make a Hat. Maybe she makes a sort of holodeck spell to "play" in Hogwarts and it takes over the whole town because Hellmouth. Sky is the limit here.

Anyway the sorting should be by their personality. The only must-haves are that Spike and Buffy are Hufflepuff and Willow is Slytherin. (Ima post my personal interpretations of their Houses below but those are "use these if you want/agree.) When Spike and Willow are Sorted, people start to think the Sorting Hat is broken. Willow decides to either "prove" the Hat is broken and she belongs in Ravenclaw and Spike in Slytherin, or "fix" it to change their sorting. Things Go Badly, but eventually HEA and Spuffy happen. People are probably surprised Buffy is Hufflepuff and not Gryffindor.

Even Spike is surprised at his Sorting and protests "I'm evil" etc until Buffy is sorted into Hufflepuff and then he's either smug about it depending on if Angel is there or he subsides to token protests.

Must Haves:

Buffy and Spike are Hufflepuff and Willow is Slytherin

Willow tries to "fix" things and it blows up in her face; proving herself a Slytherin

Spike proves his Hufflepuffness

HEA

Can Have (or use your own):

Tara as Hufflepuff

Dawn as Ravenclaw

Giles as Ravenclaw or Slytherin (depending on where he is on the ruthless scale at the point of your story)

Xander is a squib

Wes and Cordelia as surprise Gryffindors

Angel is a Slythern but tries to tell everyone he's a Gryffindor

Anya as Slytherin (but would have no temptation to join Voldemort; she'd be pursuing her own agenda thank you)

Oz as potential Hufflepuff

Jokes about Buffy's name and "Hufflepuff" such as HuffleBuff and Bufflepuff

(I think Buffy is a Hufflepuff because she's so focused on others/self-sacrificing, in both her personal and slaying lives. (See how she pretends to be OK in S6 to protect her friends/because it's what she wants.) Gryffindor has the protector thing alongside the bravery but less so the people thing. Spike is Hufflepuff because he's a people-person, too, and molds himself to the people he cares about. Also, Hufflepuffs are often vastly underestimated, which is true of the both of them.)