ilovemadrid, they won't know. The problem may arise when you check in. The airlines ensure that all passangers aboard the plane have propper visas, etc. so they aren't faced with returning the passanger. "Most" airlines will let you slide with a one-way ticket but I've encountered a couple counter agents who have required a return ticket. Be prepared to buy a one-way (return) ticket when you check in. If you have a credit card handy you can purchase a very expensive refundable one-way and cash it in once you arrive at your destination. I've had to do that twice before, once with BA and also with Air France. On British, I had to purchase the one-way ticket and when I went back to the counter to check in, the new agent didn't bat an eye at my one-way ticket! I had just wasted a half an hour and a $2000 temporary credit card charge for nothing. If you have problems and there are plenty of check-in agents, you could go to another one and see if you get a different answer! It really will be in the hands of the counter agent.

Marcia, does your daughter have a student visa? If so, she'll be fine. It's very common for students to travel on one-way fares. If she doesn't have a student visa, see if you have time to get one. There is paperwork involved so you should start early. Since she's going on a program my guess is they have arranged for student visas.