Teen girls spend a lot of time in their rooms. Sometimes they're being sociable – there's a lot of networking going on. Sometimes they're watching clips on YouTube or listening to bands or experimenting with hair dye.
Basically, all of this means they're keeping up with what's new. They know to the minute what's bright and shiny and what, rather sadly, is mouldering on the reject heap.
This makes present-buying extremely difficult. The high street is your friend, especially if shops have good policies on refunds and exchanges.
But you must also never forget that nearly all teenage girls
a) like perfume
b) appreciate posh notebooks (weird but true)
and c) will never, ever mind if you decide to chicken out and give them cash...