You cannot abandon a quest you have accepted. You must complete the tasks you are given in order to remove them from your Quest Journal.
It won't take you very long, particularly now that you are higher level, to complete those quests in other worlds and you may unlock more quests that are needed to obtain spells and pets for your school.
Sorry, but Professor Greyrose is right. There are certain quests that are required in order to gain access to pet and spell quests. There is a short string of quests in Mooshu that will prevent you from gaining access to others in other worlds if you don't do them.
As she also stated, for a lvl 70 those remaining quests in Mooshu should be a breeze and would not take long to do.
The same is true also for doing Grizz/WT. If you don't do the spell quests there, you can't access the ones in a later world.
Life Theurgist