A few thoughts. You're quite right, a school board is not a monarchy or even an oligarchy...it is subject to the voters. What you're implicity endorsing is a "monarchy", or really, a tyranny of the judiciary. A judge is not a monarch, and yet the judicial branch in recent decades has acted like one...and, like a monarch, a judge cannot be removed except under the most extreme cases...in essence, a judge is a judge for life.
Secondly, the Constitution addresses the prohibition of Congress (not a school board or a state) passing a religious law. So, even though the judge used the Constitution as an excuse to insert his curriculum preferences into the town's schools, such an argument is not germane.
As for being sued, anyone can be sued for anything.
I am appalled that a federal judge is deciding curriculum. The people of the city or school district have that job. Nowhere does the Constitution give the federal government ANY right to decide what local schools teach.
When you get around to reading the judge's opinion, you will see that he basis his jurisdiction on a long line of cases that apply the First Amendment to the states. I don't like it either, but it's based on the 14th Amendment. It's Abe Lincoln's fault.
Also, when you read the opinion, you will learn that the judge found (as the plaintiffs originally complained) that the school board's activities violated the state constitution too.
As for being sued, anyone can be sued for anything.
There you are. Do something stupid, deceitful, and unconstitutional and you might be sued. You might lose. That's what happened here.