I declare shenanagins
A big, dramatic way to say someone did something dishonest, and you're not going to let them get away with it. You shout it in the middle of the cafeteria so everyone hears.
I declare shenanagins! You told my crush I had a cow for a pet!
I declare shenanagins! You took my favorite toy and left me with a rock!
I declare shenanagins! You said I was the worst at math!