It's not the plant. It's you. It's your responsibility to get the work done, and for whatever reason you're not doing it when you should be. That's a motivation problem, not a marijuana problem. You're the one making the choice every time you put down the books and pick up the bong. Don't blame a plant for something that you chose not to do. It has been proven in experiments that its effect on memory and concentration last only as long as you're high, so you failing the test on Monday had absolutely nothing to do with you smoking up on Friday and everything to do with you not putting enough time/effort into learning the material.