Hi Esperanza!
Prepaid debit cards are great for students and also as a backup for your own ATM card if you're worried about something happening to it and then getting stranded. You can set the amount you want on it, and you can add funds as well. They work with a PIN just like regular ATM cards. AAA sells the Visa TravelMoney Card, and American Express sells the TravelFunds card. A lot of people have gotten the Visa TravelMoney cards at AAA for my trips, and they work very well. The one thing you MUST do, however, is activate the card in the United States before the trip. Otherwise it won't work.
AAA Visa TravelMoney:
http://www.aaamidatlantic.com/financial/vtm.asp American Express Travel Funds : http://www10.americanexpress.com/sif/cda/page/0,1641,18622,00.asp
If anyone in your group takes a regular ATM card and/or a credit card, remind them to call the bank or credit card company to let them know that they'll be using the card in Spain, and the dates. For security reasons the card may be frozen when the bank/credit card company notices a change in country. I've seen it happen!
Here's a good article you can share with the parents from Rick Steves:
http://www.transitionsabroad.com/publica...ards_cash.shtml