Riddle 49

In a little old school-house that stood on a hill
A little old schoolmaster taught with a will.
But over his pupils he had no control;
They said he was crusty and cross and my whole.
And the rascals declared it would serve him just right
To play him a practical joke some fine night.
So down to the river they went, and they took
My first from my last of the dark, muddy brook.
Then they eagerly hurried, yet still as a mouse,
Till they came to the little old schoolmaster’s house.
They smuggled my first in my last with great glee,
And chuckled to think how irate he would be.