The Gryffindor-Slytherin rivalry was a perdurable one. Had you asked any of the Gryffindor or Slytherin Heads of House in the past seven centuries, they would have told you that there was simply nothing to be done about the rivalry. Nothing at all.
Not to mention, they themselves participated in the bitter competition.
Absolute nothing helped. Forcing Gryffindors and Slytherins to have class together only made it worse. Making them work together helped not at all. Even though Gryffindors occasionally dated Slytherins, it had no effect on the overall behavior of the two Houses.
Minerva McGonagall had often despaired of the nastiness ever disappearing, even in the face of a common threat like Voldemort.
Severus Snape didn't much care whether or not the conflict ended. It had been going for years, and it seemed it would continue to.
Besides, they both fueled it.
No, the rivalry would never end, they had decided back when Harry started school.
Especially since Harry had started school, and clearly he would earn his house many accolades.
Making the bitter conflict all the more bitter.
Minerva didn't even understand where this hostility came from. Gryffindor and Slytherin had been good friends. Their students didn't have this stupidity between them.
Maybe it was a clashing of personalities.
But two Houses couldn't possibly contain so many flammable types.
Didn't matter, really, with the ongoing tricks and challenges.
Until, that is, Harry Potter came back to teach.
It's not as easy as that, of course. Albus died peacefully one day, making Minerva Headmistress.
And someone was needed to fill Transfiguration and Head of Gryffindor quickly.
Thank Merlin that Severus' husband was available.
Harry sanguinely took up the mantle of Head of Gryffindor. Everyone expected him to further the rivalry, encourage his House, become the silent leader in the competition.
Harry turned around and told his House that this idiocy had gone on long enough and that it was to end.
The students gaped at him. "End? The seven-hundred-year long conflict? Absolutely not."
They argued, yelled, debated, screamed, threatened, but Harry stood strong. "No baiting them, no fighting, no dueling, no teasing, no nastiness. Treat them with respect and courtesy."
"They'll take us for wimps! They'll take us for losers! They'll declare victory!"
"Don't care. You are the stronger for ending it peacefully even though you may be treated like the losers."
They weren't, of course.
Because Severus was telling his House the same thing.
His Slytherins were even more outraged. Their Head of House had loved their pranks against Gryffindor for years. Had encouraged them.
How could he say now that they were to stop it?
Severus was firm (and intimidating). "I will take points for it. And give detentions. And you know what my detentions are like. Not to mention, I'm sure Professor Potter will enjoy giving you detentions as well for playing tricks on his lions."
It worked. Now the only perdurable rivalry between Gryffindor and Slytherin was which Head of House would top that night.
Perdurable – adj. Very durable; lasting; continuing long. Word of the Day, February 13, 2008)