Ugh, FINALLY. After running all over the monastery giving gifts for months, I've finally got all* the students outside of my class up to a B support. It is technically possible to be finished recruiting before winter sets in (i forget the month, the one where you go to Remire).

*I say all, but Leonie's B support is time locked... probably until
Spoiler
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Jeralt's death.
Similarly, both Ferdinand's and Linhardt's B supports are time locked as well, so you can't recruit everyone until certain story events happen.