In for it

when her ears were bent towards him, and I felt her form beneath me gathering up like whalebone, and her hind-legs coming under her, and I knew that I was in for it.

"You sure did [hit someone with the snowball]," added Teeter, stiffling a laugh. "And of all persons in the school but Professor Rodd. Oh my! Oh wow! You're in for it now! He won't do a thing to you fellows! Look at his hat! Here he comes!"