In line three of the instructions, we are told to use the list 'class' (which contains our student items). This is misleading, as there does not seem to be any such list called 'class'. Using 'students' instead seems to work (in my case, using 'class' resulted in the item 'alice' returning the wrong average, even though my code is correct, as evidenced by my completion of the lesson after switching 'class' out for 'students').
If anyone else is getting an error message, make sure you're not using 'class' in your for loop. That might solve your problem.