There are good reasons for arranging your overseas adventure with a nationally-known educational travel sponsor.
Mainly, you can be sure that the trip will occur, and pretty much in accordance with your expectations and the promises made to you by the company. (That doesn't always happen with a tiny new company strapped for cash, or with a local travel agent doing quality student travel arrangements for the first time.)
Your best bet is with an established company like passports. Here's why:
a track record involving tens of thousands of very happy campers. The best word is word of mouth. Ask people about passports.
a special bank depository account in which trip payments are held, until needed. This protection is offered as a matter of contractual agreement between the company and its travelers.
health and accident insurance provided to group organizers, free. You don't pay for your Motrin in Florence, or for the flu shots from the doctor in Athens, or for anything else along those lines.
bonding of all providers of travel services. The buses all show up on time, at the correct hotel, or there are consequences.
a liability insurance policy covering group organizers, teachers, and chaperones. This is the sort of thing your school superintendent wants to hear about, and the sort of protection you deserve as a professional educator.