Honest opinion is do not give a cent to a cabin crew prep academy. It's absolutely unnecessary, and entirely useless. Most people who get jobs as cc have never done one of those courses. As for the high school thing, check the individual recruitment sites of the airlines themselves. Unless they state it's essential, you will be fine.
just checked the virgin requirements and it seems youre fine.
You will however need to demonstrate....
Maturity, especially important given your age but I know they have hired that age group before so it's not a deal breaker.
Face to face customer service skills.
Ability to turn up when required and on short notice, yes they do call at 3am to tell you, you've got a shift.
Reliability, no sickies because you drank too much the night before or were partying etc....
Preparedness, make sure you have all required documentation before you apply.
They are hiring you to deal with people, not write an essay, so focus on how well you can do that.
competition is stiff though. There are many FAs returning from the Mideast with great exp in inflight service and supervision. doesn't mean you won't get a look-in as airlines do want fresh faces as well. Keeps motivation online high o ave people excited about the job.