The options the school will vote on are Bees, Icebergs, Hilltoppers, and Wombats (pg 49-50). By Christmas, the votes are in: with only 32 votes, Wombats wins the place as Merryweather's new mascot. 

Not for long, though. At the beginning of the third marking period,

"the Wombat is dead. No assembly, no vote. Principal Principal made an announcement this morning. He said hornets better represent the Merryweather spirit better than foreign marsupials, plus the Wombat mascot costume was going to suck money from the prom committee's budget" (pg 95).  

Of course, this mascot comes with more controversy too. The PTA started a petition to change the mascot again after hearing the cheer, "We are the hornets, the horny, horny hornets" (pg 141). This final time, however, the mascot doesn't change. Instead, the Honor Society writes a counter-petition talking about lack of identity and psychological harm.

While all this change seems silly, it illustrates a few larger points about the novel. For one, it shows how adults judge and stifle teenagers. The mascot is always dangerous to the various adults in some way - too sexual, or too demonic, or too weird. While most students don't care one way or the other, the adults seem worried that the mascot will inspire students to do something terrible. The only time the students want to change the mascot themselves is actually for what could be viewed as a good cause: they want to respect an endangered species. Teens seem to be better than adults give them credit for. 
Another point the mascot debacle shows is the lacking an identity is harmful. Even though Melinda scoffs at the whole mascot debate, she herself is a girl without an identity or clan, and it is absolutely psychological damaging to her. As silly as the whole thing is, it speaks to Melinda's larger problems.