Sure it is our place to criticize her decision, she tweeted about and told the public... If she wanted to keep a secret and not have her decision vulnerable for scrutiny then she should of kept it to herself.
The being said, it is a very big deal right now how she is qualified for finals and is in position to potentially win the event. That is not something that happens every 8 months like you alluded to in your post. Look at Nick Geopper, Tom Wallisch, both those guys are on point but didn't make finals at x-games this year due to weather/injury.
This sport has a lot of variance, hence why we always see a different winner in slope style each year at x-games. It was foolish for her to throw away this opportunity to go and take a high school exam, when it is clear that her dream and ambitions is to be a professional skier. She is essentially right now throwing away the opportunity to live her dream, to instead follow through with a backup in case skiing doesn't work out.
If her dream was to be a doctor or something like that, then yes I completely understand why she choose what she did. But you don't get to where she is in this sport right now unless you are truly dedicated to your dream. And right now she just blew away her dream to go take a high school oral exam...