Wow. Why would you call me that when I asked a serious question? No one suggested first-graders packed heat. But I don't for a minute think that the terrorists would care one iota about children if they were gunning for Bush. Please be civil.
Newbies should read listen and learn before posting you are a troll and a disrupter welcome to the kitty litter sand box
You've been provided an education by others who have phrased it better than I on this question, but I trust that you understand why a first day member putting up something that could easily be seen as an incitement might generate suspicion. Whatever the outcome of this thread, hang around a bit, or simply do a search on the word "zot" and examine some of the outrageous things that get posted by trolls doing precisely what some suspect you of. It's usually pretty offensive, not to say stupid.
As to your specific question
What is a good response for those who question why the Secret Service did not immediately remove Bush from the classroom on 9/11 when, at the time, it would have been reasonable to consider him a target?
At the time there was nothing to indicate any threat at that location. In fact, there would have been reason to believe that the particular location was as secure as anything they could get to in a reasonable amount of time. When a problem occurs the first thing is to gather tactical information unless there is an immediate threat.
There would have been no way to tell that any theoretical threat might not be outside rather than inside and, in fact, that anyone threatening the President might be counting on the President being immediately moved.
You don't jump until you know that it's not more dangerous to be "there" than where you are now.
Options would have to be assessed. Would it be better to put him in the car, traveling along limited known roads, or bring in military helicopters, for example. And where could those come from? And where would the nearest truly "secure" facility be?
The only known threat was in NY. He wasn't in NY. Raise the threat level to "outrageous" and gather intel. It's real simple.