Technically they have three schemes acting together:
1. Passing grade dishes taught in a secret class, to ensure only the kids they want to pass know exactly how to pass.
2. Choice of ingredients out of season, to ensure the loophole that students can use their own ingredients is subdued.
3. Giving bad ingredients that will always produce lower quality results.
That and the fact that the best students are grouped together, to create inequality in the comparative grading system, and the time limit, that hinders the ability to make a bad quality ingredient better. It's a good plot, even if it feels unfair. It does reveal that Azami sees them as threats to his system, and that he is not right. If his system was inherently the perfect one, no sort of alternative cooking would be even considered a threat. He might have to meet these rebel cooks directly, but, for now, cheap tactics are enough of an atempt to get rid of them easily, without revealing his "fear" too much.