This applies not just about going from Rookie to D class, but also from D to C, C to B and B to A class.
It's all about how many corners you do per number of incidents aka - CLEAN SAFE DRIVING
Where you finish or where you place as absolutely NOT bearing on moving up license levels.
A quick introduction to the SR system:
https://forums.iracing.com/discussion/675/%EF%B8%8F-an-introduction-to-the-sr-system#