How does Anne describe the class quaking in boots?

How? The class was "quaking" in their boots waiting to see which students would be promoted to the next grade.

From the text:

Our entire class is quaking in its boots. The reason, of course, is the upcoming

meeting in which the teachers decide who'll be promoted to the next grade and

who'll be kept back. Half the class is making bets. G.Z. and I laugh ourselves

sick at the two boys behind us, C.N. and Jacques Kocernoot, who have staked their entire vacation savings on their bet. From morning to night, it's "You're going to pass, No, I'm not," "Yes, you are," "No, I'm not." Even G.'s pleading glances and my angry outbursts can't calm them down. If you ask me, there are so many dummies that about a quarter of the class should be kept back, but teachers are the most unpredictable creatures on earth. Maybe this time they'll be unpredictable in the right direction for a change. I'm not so worried about my girlfriends and myself.