Hola maestra! I have been taking kids to Spain for the last 8 years and I would say your girls would be fine in the summer shirts you describe. If you want to know what is being worn in spain go to the corte ingles, mango, zara web sites (I have all the addresses at my school...but a quick search on
www.google.com should turn them up.) We are from North Dakota and not used to the heat of southern Spain so we feel the NEED to dress as briefly as possible while wanting to maintain modestly. My girls and I always bring sun dresses (short or long in length, spagetti straps to capped sleeves) which are awailable allover (JCrew to Target price range), they bring skorts and skirts with a rainbow array of the little tops to which you refer. Have your girls bring the minimum and then encourage them to go to some of the teen stores and buy the very clothes they will see the Spanish girls wearing....a great opportunity to practice the language in context!!!..OJO! as has been mentioned, if any of your girls are regordetas or even much over size 10, they will have trouble finding clothes to fit, so if this situation exists you will need to be sensitive.
We have only been 'kicked' out of the Almudena in Madrid once (after many visits...ususally in similiar dress) and it was I who wore the offending spagetti strapped dress. I DO usually tell the girls on the days we visit cathedrals to wear a skirt or dress and not have their bellies (or anything else) on display.
Ususally we have one member of the group carry a 'group tee shirt' so if someone's top/dress offends s/he can just slip the shirt over top and we can proceed.
Where are you and students going in spain? For how long? Maybe our trips overlap! I'm leaving on 9 June and return 29 June. We do Madrid and south. Send me a private emilio if you want to compare 'notes'
Hope this will help!
perfecta