It could be a device or browser issue. You might figure out what devices they are working on and what browsers and find the same for a random sample of people who complete without a problem. See if there is a pattern. If they are issued by your school you might check with IT about permissions...maybe something has gone wonky.
They ARE enrolled? Is it possible their connection dropped? Completion shouldn't work with no connection with Lesson, but i'm sure stranger things have happened (this is a frequent problem with SCORM). It is a screen shot of their account, not someone else's? I know, I sound like a really suspicious so-and-so...15 years working in the prison system resulted in 0 trust in anyone (and that included staff). Feel free to disregard my trust issues if you like.