The disciples knew He was using the unleavened bread of Passover to show His death as the actual death prefigured in the Passover. The Passover lamb was slain, the blood was poured out upon the earth and a small portion was spread on the doorposts and lintel as a sign for the death Angel to pass over that home. The Levitical law was not violated by drinking some of the blood. That's Paganism.
Then inside the Home the sacrificed lamb was eaten, all of it, leaving nothing except what would be burned up to finish it. Had Jesus been offering His real flesh (which He had taught them but apparently not you was going to profit nothing in eating it) then He would have been fully consumed or burnt up in closing the Passover meal.
But Jesus had not yet been slain the night of the Passover Seder, so He would not have offered eating of the passover actual lamb slain from the foundation of the world because He was not yet slain. So what do you think He meant by 'this bread is my body to be sacrificed for you'? ... He and the disciples left the Passover meal and went to the Mt. He was not burnt up as the Passover actual sacrifice was commanded to be dealt with if there was any leftovers.