Found it on soc.culture.sri-lanka ….

The dean of Engineering had once walked into a class, and said
Good Morning. The whole class chorused Good Morning.

Hi, you are freshmen arent you? he asked.

One student bolder than the others asked him how he knew.

Well, he said. When I say Good Morning to a class, if they are
freshmen they say Good Morning too. If they are sophomores, they
quietly fold their papers away, and look at me. A class of Juniors
will look at me over the top of their papers, and then get back to
them. A class of Seniors will ignore my greeting, and keep reading
the papers. When I say Good Morning to a class of graduate students,
they write it down.