I suspect most teenagers (and that's what we're talking about here - 17, 18 years olds finishing secondary school) would have a notion that jpg and gif are image formats, and pdf and dcx are a document formats. I suspect few would know much beyond that, and most would not have had much reason to worry about converting between formats. [I work in higher ed tech, my gut feeling here is based on performing usability testing of other software]
Add in an unexpected file format glitch during the most important test of their life? Yeah, I'm not at all surprised that some/many screwed up.
This is 100% on College Board for failing to handle the situation gracefully. They didn't need to accept heif files. They did need to provide detailed instructions ahead of time, including possible issues with unsupported formats.