I would gather giving students incentives to reply honestly, seriously. Generally speaking there was a couple of basic ways to get someone to do something. Motivate them or scare them This is basically rewards and punishment. One other way is the motivation is to make so that the person or persons does something because its the right or moral thing to do. IMO I believe this principle would be true in getting students to fill out a survey. To be specific to set out benefits or threaten them. For example if anybody is caught not being honest to write a 100 word essay on how being dishonest is wrong. Or conversely an award such as praise from the teacher.