A well designed envelope is important because it is recognizable in the oceans of junk mail flooding our mail boxes, presents a favorable image of the committee, and generates a change of address report..
Your school logo in the return address attracts attention and will save your letter from being tossed into the trash. Make it BIG, so it will not escape notice, but do not make the return address too large lest the postal service's scanner reads it, instead of the addressee's, and mails it back to you.
If you place the instruction (endorsement) on the envelopes "ADDRESS SERVICE REQUESTED" the Postal Service will notify you of any new addresses and forward your mail too, for a small fee. The Postal Service enters change of address orders from patrons into their database, and for 12 months after that, they will label mail sent to the old address with the forwarding one. If you have the instruction "ADDRESS SERVICE REQUESTED" somewhere above the address, they will photocopy the envelope with the new label and mail the copy back to you. The Postal Service will charge you 50 cents for this service, but of course, you will save the 33 cents and the time that would be required to print, stuff, label, stamp, and remail it
The full nine digit Code for any address is available on the Internet. It also lists the county.
The format for the address follows the "Proper Address guide in the 1996 National Five-Digit Zip Code and Post Office Directory. The ariel font is easier for the machines to read. A comma after the city is out, one more thing to unlearn, and capitalize the state abbreviation.