These graduating students, or graduands to be technically correct, most likely from Emmanuel College, are crossing Emmanuel Street on their way to their graduation ceremony in the Senate House. It reminded me of school when we had to walk behind the teacher two by two. At the front of the crocodile you can see their "teacher" with the mortarboard. He is actually their Praelector, who is a fellow of their college. The Praelector formally presents the students during the graduation ceremony.
I would like to thank the Geograph colleagues who have advised me on these mysterious technical terms. They have a far greater knowledge than I of degree level study in a superior establishment such as Cambridge as I'm just a lowly 1970s Polytechnic Graduate.
Reality Checkpoint - so called because it supposedly marks the spot, halfway across Parker's Piece, where the bubble of Cambridge University ends and the "real world" of Cambridge city begins. I believe there used to be a plaque inscribed "reality checkpoint" on the lamp-post, but that is long gone and the words are now rather crudely scratched into the paintwork.