Take a look at the passage again:
As waves of boring information rolled from Professor Clark's mouth. Kris
grabbed his mental surfboard and spent the rest of the hour riding fantasy
waves.
You chose to alter the sentence so that it began with a main clause followed by a subordinate clause, like this:
Waves of boring information rolled from Professor Clark's
mouth, while Kris grabbed his mental surfboard
and spent the rest of the hour riding fantasy waves.
Unfortunately, you don't need a comma to connect a subordinate clause to the main clause in front. Review the rules.
Go back to the sentence to try again.