What happens when you reenlisted to reclassify, but you reenlisted as a SPC and picked up SGT before your school date?

Generally speaking, you should keep the MOS you are going into.

It can depend. In my MOS at one point, SGT(P) were supposed to drop their promotable status before attending AIT but after course attendance approval. Some apparently didn't do this and as a fallback, the schoolhouse was supposed to remove the status. Well in some cases the promotable status was never removed by anyone, so many got promoted to SSG under their old MOS, and were still allowed to graduate and go to their first unit as a SSG. Mind you, we were not accepting SSG's for recruitment/reclassification, because we were overstrength on 30-level.

If it were conditional that you not be promoted you wouldnt have been able to reenlist to it if you were promotable