It's free to play. Any money you spend is optional. The game is designed in such a way that the free characters are still viable in all game modes (just not in the few instances where you can optionally increase the difficulty for no extra reward). And if you do pull for the characters, you get them at full power: dupes don't matter and there are no weapon banners or relics to optimize.
The only thing is that it's a tower defense game, so if you're not into those, that'd be the only reason I would not recommend the game.
We all know you're in high school