Working at a school, surely you have your "emergency excuse mode" well tuned. Tell them that you split your coffee all over your trousers when the lights changed, but fortunately you had these (hopefully smart) jeans in the boot of the car and thought they would be better than trousers with a...
If you think quick enough, the answer is that you have a swollen toe and you decided matching slippers were better than one slipper and one shoe.
(Or as an alternative, get plain black slippers instead of those pink fluffy ones with bunny ears!)