Aw, I feel for you both, OP!
And, as a secondary teacher, I am familiar with the scenario!
Despite all the swagger, she must be a bit disappointed with herself. These are complex emotions for young teenagers to process or show. If she is like 99% of others I have seen in this situation, she will be feeling very insecure but desperate for someone to blame for not fulfilling her dreams in the future. ('It's not fair, I could have been a big pop star if...')
BEWARE! This is a role you don't want! 
My advice is to start from the end. On the surface, be 'supportive'. But do not do anything proactive for her. IMO, your job is to help her along her own way now. But let her come to the conclusions of what she can or can't do herself.
This is really important to an immature little adult. You saying 'but you won't get a visa for LA! Yuo need to do this instead!' translates as 'You can't do this! You have to do what I want you to do!'
Any teen worth their internet access will then respond with 'I'll show you! Stop ruining my perfect future!' 
Instead, start at the end together. Right, so let's see what you need to be a big star in LA! (wrestle the sarcasm out of your voice, btw!)
How much is a flight?
How do you get a visa?
Where are you going to stay?
How do you get an agent?
etc.
Let her research this - you help her if you can. Act as if it is actually going to happen. Then she has to 'work backwards'. So a flight is £700? Right, write out your plan of how to save this. So you need a job to get a visa? Ok - what kind of job? Etc.
She will eventually arrive at the same conclusion as you! She needs some qualifications or proof of discipline/achievement. She needs money. She needs a job. She needs a specific goal.
Then it is up to her to sort them all out, with you cheering along from the sidelines!
A maths teacher and I (English) had to conspire this way with a kid who was adamant he didn't need his GCSEs because he was going to be a drug dealer. 
He is now a mechanic, with a GCSE in both subjects.
Good luck!!