Here is the original item:
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 begins 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.