Take away the computers and they’re just sitting ducks.
A play on the phrase "sitting ducks" --- figuratively, sitting ducks are vulnerable people. The joke from the professor is that, if computers were taken away from their students, they would be incapable and vulnerable, a reference to dependence on technology. But, because the students are literally ducks, if you took the computers away, you would literally be left with a room full of ducks sitting down.