The figures for students graduating primary school (18.57 million) and entering junior high school (18.49 million) match up quite well, though. Do you think primary schools and junior high schools manage to coordinate massive student number inflation to the tune of 3 million non-existent students, but then at the transition to senior high schools that suddenly breaks down? If anything, I'd expect it to break down when those non-existent students are supposed to take the Zhongkao exam in order to graduate, not at the senior high school admissions stage.
Some statistics reported in China are unreliable because the person doing the reporting also has their performance evaluated by the numbers they report and there are few external checks on validity, but I don't think that's the case for student numbers in particular.
Also, it seems like you're the same 'jldugger who cited Chinese population statistics upthread, but when somebody else does it, they're suddenly unreliable???