I.C.
You don't have to do anything that you don't want to. It's up to you whether or not your give the school any time.
Having said that, I do think that your work is very valuable. Not only do you raise money for the school, but you also communicate to your daughter that she is important, that education is important, that books are valuable and that helping is a good thing. These are all lessons that can best be taught by actions, not by words.
If you chose to continue volunteering, you must resign yourself to the fact that appreciation is not going to be forthcoming. You can respond to this by building in little appreciative things for yourself during that time (allowing yourself to order dinner out one night, arranging for coffeee with a special friend after the fair, etc.) or by instituting an appreciation event at your school (yeah, I know: more work). After all, you are probably not the only person to feel ignored you have the added burden of donating a professional skill that used to be a source of income for you!
You are in control here. You can chose how you spend your time -- and how you feel about that time.
Remember your decisions on (1) how you invest your time and (2) how you FEEL about that choice are being communicated to your daughters. If you chose to continue running the book fair, try to do it cheerfully and willingly.
I.
PS I've done a lot of volunteer coordination. I've learned that volunteers work primarily for three reasons: (1) public acknowledgement (2) gifts and/or plaques and (3) internal motivation. As a volunteer coordinator, it is important to reward each volunteer in all three ways to be sure that you hit the right one. It could be that your principal is the kind of person who is satisfied with the internal feeling he/she gets from volunteering and doesn't understand the importance of meeting the other two needs. You might need to educate him/her on this -- it could help with all kinds of volunteer recruiting.