It's bad enough they passed some legislation a few years ago[1], but the damage has in many senses already been done. And it's unclear how effective the changes will be. So it's entirely possible those 3 million missing high schoolers never existed.
[1]: https://www.reuters.com/world/china/chinas-top-legislative-b...
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???