One thing we did is pointing participants to your awesome iNat or iNot quiz! Thanks so much for putting that together.
I think another thing that happened was that the largest class participating this time around had already been exposed to iNaturalist for a class project before the BioBlitz. (I have to check but expect a lot fewer new accounts created right at the start of the BioBlitz.) We tried to tie this to an actual research project to make it more relevant and not just a class exercise. This was an in-class project where students made annotations and generated phenology graphs on iNaturalist under instructor guidance. So students out of that class who chose to do the BioBlitz for extra credit had worked with the website before (not just the app on their phones) and were more familiar with the concept of generating or cleaning up data for use in actual research. I think overall there was a stronger understanding that iNaturalist observations represent raw data for research and thus quality is important. Some of our top contributors were bio students involved in research projects.
In summary, I think framing it as a research project and not just a competition, as well as giving students prior exposure and a behind-the-app look at iNaturalist and what else can be done with the data beyond asking the questions “what plant is that?” has helped.