What does the sentence "four class emblems" refer to? (Please, see context)

Dear all,

I'm a Spanish student trying to read this book ("Tempted") in English. I read the firsts ones in Spanish some time ago. Now I seem unable to understand one sentence.

It is in chapter thirty-eight, on page 295. Zoey is describing the cathedral where the High Council is holding its meeting and she says:

"The stained-glass window scenes of the original cathredal had been changed from bloody Jesus on the cross and a bunch of Catholic saints to a representative of Nyx, arms upraised holding a crescent moon between her hands, a brilliant pentagram close beside her. In the other windows I saw stained-glass versions of the FOUR CLASS EMBLEMS that symbolized which year a fledgling ranked at the House of Night."

I don't understand what "four class emblems" refer to...

Can anybody help me?

Thank you in advance!

Laura