Welcome to the cloud, where tracking cost is very important. And Azure does not really care if it is production or training. (Although you can have some student account with some free credits.)
When you created the service, you should have been shown estimated costs. They should have deterred you. If you have not done so already, delete the service, or else the cost run away even more.
Myself, I occasionally create dedicated pools in Synapse for testing, but I always draw the bar far to the left to keep down the costs. And I typically delete it within the day, since else the pool will eat up all my Visual Studio allowance, and my subscription gets suspended for the rest of the month.
Let's hope that Azure support can be kind with you and cancel the debiting for you. 500 USD is not a small cost for a student.
But be more observant about the cost in the future.