Only a parent would know about class projects sent home with 3rd or 4th grade students for parential help to support one cause or another..
These classroom projects for mailings are usually written in capital block letters by the child. The parent most likely emulated the kid's method of printing as he addressed the envelopes, unlicked ,but with the flap folded in , to avoid self contamination, and specially purchased envelopes with a self embossed stamp. How many foreigners are aware of those?
Also Arabic is written left to right ,and horizontal, a habit not easily broken - the fluency ,and the printing slanting downward is strictly another clue of home grown writing ....
as jh2 says, my 3cents worth.
As you point out, the 4th grade class reference wouldn't be meaningful to an alien in this country.
The use of prestamped envelopes and unlicked flap indicate someone who's watched all the Discovery or TLC detective series where someone is nailed by his DNA!
The "09-11-01" at the top of the letters is trying too hard to pin it on OBL and company. I don't Osama's boys would feel the necessity of identifying themselves if they were the anthrax perps.
Finally, the misspelling of penicillin as "penacilin" in one of the notes (Brokaw, I think) indicates to me that this is a native english speaker. If I were going to write a threatening letter in some language I wasn't very fluent in, I'd pull out the dictionary to look up the technical terms. It reminds me of Internet-speak, where few authors bother to get the spelling right but just type it out the way it sounds.
And what's with the "09" for September? I've seen a lot of references on FR and elsewhere to the events of 9/11/2001 but I don't recall anyone writing it as "09-11-01".
The various eccentricities about these notes are eventually going to nail this guy.