Alright, here is my logic. Basically, it falls under three categories.
1.) History. We only put it into the pledge when we were fighting communists so that we could show that we were different than the, I believe an exact quote from people like McCarthy at the time was, "Godless Commies who want to steal our way of life from us..." but I may be mistaken. That was why it was put into the pledge of allegiance. It was not put in to show taht we have a history of religiosity, or to show that our founding fathers were religious (which many of were not, but that is another topic entirely), but merely to fight an enemy. That serves no historical or even practical purpose now. Thus, in a legal argument, it would hold no ground because a lawyer could not argue that it had been in for 200 years and thus showed our religious roots when all it was meant to do was fight an enemy.
2.) Direct Funding. This is honestly the weakest argument of the three I have to make. This is because it is hardest to prove that all students in the country are forced to say all parts of the pledge, though it is common practice (for those of us who went to public school) to be forced to say the entire thing or suffer detention-and please, little kids don't say anything about it, no matter how much we would like to think they do. I would love to think a daughter of mine would tell me if a teacher forced her to say it, but I doubt that would happen because it is just common practice from what I have seen. It is also hard to prove because it happens at the beginning of the school day, sometimes even before school begins when teachers are technically not paid (thus no taxpayer money...). As I said, it is the hardest part to make, but if you want, I will try and make it....just honestly don't have the patience at this point.
And 3.) Seperation. Probably the easiest to prove in front of the liberal 4 of the Supreme Court, but harder in front of others. In Lemon v. Kurtzman, the court set up a test to test out what was constitutional and what was not.
First, the statute must have a secular legislative purpose; second, its principal or primary effect must be one that neither advances nor inhibits religion; finally, the statute must not foster "an excessive government entanglement with religion." (This is a synopsis of what the test is)
Under God does not serve a secular purpose at all any more which is proven under the history section, but if that argument does not fly with you, then there is another one. Under god also doesn't serve a legislative purpose at all. Think about it. What does it serve...that's right. After the first one is proven, the rest don't really matter because it is an all or nothing.
But Lemon also allowed for some entanglement of church and state which meant that under god might be theoretically ok if, I believe, it matched up with history, which is a main thing for justices lately. Look, a lot of justices lately, have been going back to original intent of the founders...it wasn't there. That simple.
Dissect this, have fun with it, and I look forward to your comments because this is one of my favorite topics.