This union can go pound sand.  If the local communities and the people that actually attend this school have any real desire for a name change, then put it to a democratic vote.  Some union wanting to look proactively politically-correct by jumping on the painting-over-anything-unpleasant can look elsewhere.  Maybe if the union heads that suggested this went to school and took some history lessons, they might realize that history is made up of lots of  good and bad events, and the classification might change over time.  Sir John A. Macdonald was Canada's first Prime Minister, a significant politician that helped shape Canada.  He was certainly far from a perfect person, but he accomplished a lot more in his lifetime than the entire ranks of the teachers' union ever will.