We just got back from England and had applied for the student 11 - 15 Oyster Cards a month before we left. You can't get them once you are there if you are a US citizen. They are worth the trouble b/c they travel for maximum 1 GPB per day on the tube or bus. They preload it with 5 GBP and didn't charge the usual 3 GBP for the oyster card. We had a little trouble uploading the photos the correct size, but calling the help line, they gave me to "Johnny" who was very helpful as to what size the picture needed to be to upload correctly.
We purchased our adult oyster cards when we were there. You have to pick up your teen cards when you get there and show their passport. You pick the station you will pick it up at. We just put the 20GBP on an oyster card when we got there. and used it for 5 days with 4GPB leftover now for our next trip. Some days we max'd our adult cards out and some days we didn't. I liked this better than the 4 day travel card as we got the build in benefits of the travel card max with the low one trip fare of GBP1.50 from the Oyster Card.