You're certainly right about that. And I speak from experience. Most union leaders I've known are ideologically leftist. But the school administrators are more opportunistically leftist. The union leaders want to change society. The administrators want to get bonuses and promotions.
But little of that applies to the rank-and-file. In my school (inner city) we had a group of teachers who'd go out campaigning for Palin after work (2008). Unfortunately, union elections are gently rigged, so conservative teachers rarely get elected to leadership positions in the union.
I will inform Diogenes that I found another conservative in the public school system : )