In Evil High, the players are the bad guys, and karmically speaking, the bad guys are not supposed to win. They’ve got not only the superior forces of Good aligned against them, but severe drama within their own group as well. The deck is stacked against them from the beginning.
So how do you take an idea like failure, and present it to your players as an acceptable outcome?
By making it fun.
Let their failures not be roadblocks to the story, but unexpected detours, leading to even better results than they'd planned. Let it introduce new opportunities to shake up a plan the players thought was written in stone (or crayon).
Comedy is all about the unexpected. Your players may get the freedom to design the plan, but after they make their poster, the game is back in your hands. Surprise them. Test them. Give them opportunities to lose and fail, and through those failures, they will grow faster and stronger than you can imagine.
Please be aware that success of the evil plan is NOT part of the grading rubric. The players do not need to have this pointed out to them.