I went to an all male Catholic high school in the mid to late 1960s. All the teachers were male and many were priests or seminarians. We boys were always flipping covert "birds" at one another as in scratching our head with the offending finger. Several of the teachers would use the middle finger when pointing at the board. I can assure you they knew what they were doing. Either they were doing it back at the class or more likely were hoping to make it more normal and less insulting.

The other possibility about this picture is that it is a photoshop and is not even a church. The building looks like something else to me. There is nothing to id it as a church.