He would announce times in advance of the end of the session but everyone had to close their blue books when time was announced and put the completed exams on his desk.
So at the 1/2 hour mark, he announces "20 minutes to go."
At the 3/4 hour mark, he announces "5 minutes to go" and the 250 students are scrambling to write in their best and final work.
At the 50 minute mark, he announces "OK, close your blue books and bring them up to my desk.
All students except one do exactly that. This last student keeps on working, but is unseen in the mass of humanity working their way to drop off their exams on the TA's desk. As the last 20-30 students are putting their blue books in stacks on the TA's desk he sees the remaining student still working, and says "HEY!"
The final student calmly closes his blue book and walks forward to the TA's desk. He walks up to the front of the desk, and stands arrogantly in front of the TA. Before the TA can say anything to him, he asks in a challenging voice, "DO YOU KNOW WHO I AM?"
The teaching assistant, taken aback, says timidly "No."
The student says "Didn't think so," tucks his blue book in the middle of one of the stacks and walks out of the classroom.