Extending out your very accurate take on Mitch's genius level move is his scheming involving Julie (the sophomore girl):
--bringing her into the conspiracy as a plotter gave them a project together, avoiding awkward conversation (such as Tony and the rising freshman girl) and as the plan came to fruition, they united to pull off a legendary prank on the despised O'Bannion. . .
--as O'Bannion flunked senior year and repeated, this means that Julie's male classmates went through the torment a year before from him. She will spread the legend of Mitch's genius like wildfire and before their flame fizzles out, he will begin HS with cred not only with his grade level, but also the sophomores and of course, Pink, the cool king of the senior class.
--finally, O'Bannion's animus throughout the summer will be directed mostly towards Carl and any others that he could identify playing the prank on him. While not the brightest, O'Bannion will likely remember that Mitch was not present for the prank (he should recall pushing him on the way towards the car), and unless Carl sells him out (a distinct possibility) no one will sell Mitch out to O'Bannion and his summer should be relatively peaceful.