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1.
A derivative of the term Upper Decker, "Under the Bleachers" is somewhat less secretive and much more difficult to pull off. It is the act of taking a poop behind the toilet, under the water tank. Meant to be hidden from view while creating a terrible smell to create the illusion that the toilet could use a serious cleaning.
The dinner party was atrocious, so I took an under to bleachers in their guest bathroom.

I could tell my father-in-law disapproved of me when I entered the bathroom and found that he left a massive under the bleachers.
by ZedSpot May 30, 2010
 
2.
When someone is gone sick from school often/for a long period of time, and you don't know what happened to them. Under the bleachers means it is possible that they have bird flu, are in a spinal halo, and are taped under the high school football bleachers. You just don't know.
"Where's Johnny today? He was supposed to give me his phone number."
"He's been gone for a few days, he must be under the bleachers"
by Betty Stardust November 10, 2005