I'm a teacher and I NEVER was upset when parent's want to take their kids on a vacation to WDW. However, the beginning of the year, and especially 1st grade, is probably the MOST important time for your child to be in school. That's when the routines for the year are set and practiced and the children get to know each other and the teacher. However, now that the trip is set, go and enjoy. Just make sure that when you return you make sure to check and see what all your child missed. JMHO, I am sure others would disagree.
Besides, that......don't the rest of you know that Jersey week is in November when the whole public school system shuts down for 2 days for the teacher's convention?