How do I break off a sour friendship?

[Mr Khan]

Senior year being current, and the end of highschool being imminent?

I'd say tough it out for three months and then let the relationship wither as most high school relationships (friendships and romances both) inevitably tend to do after graduation. If there's a light at the end of the tunnel, pursue it instead of creating conflict

Unless she's intimately linked to your circle of friends. Then there could be an issue