Depends on the pair, I suppose. Because I recruited both Hilda and Caspar, and I triggered their C-support after the timeskip. Their C-support, then, was not timelocked. Ignatz and Mercedes are the same way. Student supports between each other seem to be fine, then. Byleth supports, I'm not sure. I didn't get to test it out. I mostly had B supports with everyone pre-timeskip anyway.