Jay, you are mixing apples and oranges.  Let’s start with the story of the pro-Obama textbook.  Although I must admit I generally treat anything reported in WorldNetDaily like I used to treat the stories of Bat Boy in the now defunct Weekly World News, let’s assume they are right about the use of the text containing Obama speeches.  This might be worthy of debate but it doesn’t raise any First Amendment issues.  Schools are free to purchase textbooks that make many kinds of choices about content.  It is not unconstitutional to purchase a book that emphasizes John Kennedy over Dwight Eisenhower or which gives more space to Western settlers than to the Native Americans they displaced. Again, there could be pedagogical reasons to challenge the wisdom of some books over others, but it is very rare that you can take those challenges to federal courts. (By the way, the very story you cite indicates that the book was published before Obama was a Presidential candidate and that subsequent editions removed these pages, presumably so as not to appear partisan.)

Indeed, the only ideas that public schools cannot promote are “religious” ideas.  You say, that’s unfair!  Here’s why it is not.  The First Amendment only prohibits the “laws respecting an establishment of religion”; it doesn’t prohibit governments from promoting cultural ideas, economic ideas or political ideas (that’s why we have elections)

As for the girl and the cross.  I have been hearing about cases of schoolchildren allegedly being denied their right to expression of religious ideas for decades.  USA Today famously put out a headline once reading “Girl Gets ‘F’ For Jesus Paper”.  She went to court and lost her free speech claim because she had gotten the required teacher approval for a drama paper but decided to change her mind and write on Jesus instead.  She got the failing grade because she hadn’t followed directions.

So, whether you were right (you might have been) or wrong in writing to the principal depends on some facts that aren’t on the record.  If the assignment was “draw a powerful image”, you are correct; if the assignment was “draw a representation of an animal”, you were wrong.