This will be the fourth school for my oldest who will be 16 and the 3rd school for my youngest.
Well now, let's look at the positive aspects of this. I know, I know...you're completely focused on using this as 'evidence' that he sucks as a parent, but just hang with me a minute.
By changing schools so often (and really, unless your 16 year old was going to a VERY small school that incorporated elementary, middle schoo/junior high, and high school into ONE...4 schools is nothing) your children should have excellent people skills and have a better than average ability to adjust to new places and new faces.
I attended 8 schools in 12 years. (4 elem, 2 middle, and 2 high schools). I was enlisted in the Navy and attended two different universities. I got along well with ANYBODY (still do). I adjust easily to change and am well equipped to be comfortable in just about any environment.
Maybe if you stressed the up side of meeting new people while the kiddos are able to keep in contact with their other friends, it will help ease the 'trauma' of moving to a new school.
Please keep in mind that I am in no way telling you that you should just ignore Dad's moving without informing you first. I am telling you, though, that it is better and important for you to make the best of this situation for your children.