When I taught in California, I was instructed not to fraternize with the students. Meaning, for instance, don't go out for a beer or even a cup of tea. I was used to going out for a beer or a cup of tea with my students in Sweden, so I thought it was strange. I am sure there were strong reasons behind.
I was therefore glad when Helen from our MPhil course emailed us teachers inviting us to celebrate her birthday with cakes and ginger ale right before the class.