I agree with LUV. This depends entirely on the school's policies and how much actual "paid" time is spent doing "volunteer" work.
As a taxpayer, I'd like to know that my tax money is being used wisely. However, as a parent and volunteer, I know that if the staff member is doing work that improves the learning environment, I'm happy regardless of which column it falls into.
Some people come into our lives and quickly go. Some stay for awhile and leave footprints on our hearts. And we are never, ever the same."
"The ultimate aim of karate lies not in victory or defeat but in the true perfection of one's character."
If you were working at a business would you be allowed to do non-profit work during business hours? Same situation. It would really depend on your company's/school district's policy on such a thing and what your supervisor allows you to do when you are on their time.