Mm, but it makes me doubt his judgement a little, especially after he gave one of the rapists in episode 3 an unsolicited character reference to ZYL. :-/
As jo_lasalle said, it does make sense for him to not know about the rapists being rapists and for Zheng Zhongyuan being an abuser. These sorts of things aren't exactly ones that people immediately see (especially with rapist dudes, since Shen Wei only just took on that class and had no clue about them really), and Shen Wei is on the one hand a civilian academic and on the other a Dixingian figure of justice – the evils humans commit aren't in his jurisdiction, so he doesn't need to be on the lookout for them.

Agreed that Zheng Yi is likely younger than her stated age. 10-12 is more plausible than 15, and the younger she is, the more Shen Wei's unilateral forgiveness of her makes sense.