[ghaff]

>I'd guess that students with a bad experience are more likely to reply to your survey.

Or only those with strong feelings one way or the other answer.

Or those with strongly negative feelings fear that the survey isn't really anonymous and they worry about retribution.

A. is pretty much how this would be done in most real-world situations. Make a second attempt to get people to answer and then go with what you have assuming you did get some reasonable response rate which 75% probably is.

[ghaff]

In fact, when you conduct surveys, it's very common to ask screener questions. Among likely voters, among IT decision makers, among developers, etc.

It's fairly clear in this case: undergraduate(?) students.

In general, surveys are trying to get statistics from a demographic that's interesting to the person doing the survey, such as buyers or influencers of purchase decisions for a given product.