It may be far more convenient to just eat meat, since it's commonly-available, (generally) easier to convince teenagers to eat, easier to find restaurants (especially fast food), etc.,
but as long as you are making sure to cover all your amino acids (balance!) and the teenager is willing, there shouldn't be a problem.
Ballpark says that a high school athlete (say, 200 pounds) will need around 150 grams of protein a day.
This is based off of the higher range of .82 grams per pound that I've found referenced but can't find a good canonical source for.
It's not too crazy to hit that or even exceed it:
- 2 cups buckwheat for breakfast (46 g)
- 2 cups of beans (burritos) at lunch (~78 g)
- 2 PB&J sandwiches as a snack (~24 g)
- some salad based on stuff like broccoli, quinoa, lentils, etc. at dinner (~30 g)
That's well over 150g of protein, not missing out on any amino acids, and pretty cheap. Tasty, too, if you make sure to add stuff to the burritos. It's also easier but pricier if you start adding in protein shakes.
My only real concern over this diet is the lack of overall calories... it might need a bump somewhere if this is an athlete doing two-a-days.
Of course, there may be other concerns for developing athletes, including calcium, but most of those issues are easy to fix by either providing milk
(be prepared for insane consumption... my two brothers were going through a gallon a day together)
or making sure to hit the broccoli/spinach/etc.