If you send another note to the 30 that did not respond, they are no longer randomly selected, they have become another category that took two times to respond. In fact, once you choose to do anything but use the original sample you've altered your experiment.
On the other hand, it is a fucking preferences survey sent to students so do whatever...
If the population of students that respond after one survey request is different than the population of student that respond after two survey requests (or don't respond at all), then the original 90 responses aren't representative of the entire student population. "Altering your experiment" is a distraction because the presumed goal is to learn about the entire student population.
On the other hand, it is a fucking preferences survey sent to students so do whatever...