Don’t throw it out, or pour it down the drain! Sour milk is like a buttermilk, it acts with raising agents (baking soda in particular) to make baked items particularly fluffy. If the smell of the milk is strong, then do not use it. Otherwise, bake away!
I have just made some chocolate cupcakes with outdated buttermilk. Super moist cupcakes, great structure to the cupcake as well.
Just be sure to slip the soured milk deep into the refrigerator so that thirsty teens do not try to drink it!
Peace