I'm about to graduate high school and i want to go on a trip with a friend to a beach.
Since you are in Austin both Corpus Cristi and South Padre Island are within driving distance.
Outside the US I'd recommend Grand Cayman if you want a really nice beach though if you want a wilder night life you might refer Cabo.
According to my personal views Search St. Petersburg, Florida in spain is the best beach in the world because of its sugary white sand and dolphins frolicking offshore.
I really like to visit beaches and my most favorite beach is Maldives, The Hamptons, New York and Lanikai Beach, Hawaii.
Siesta Beach and Sunset Beach!