So did our MC not already have his way with earlier classes? I thought he gets their first time (not child) if they can't beat his magic by the end of year 1 thus the sister. It also seems that the girls (at least the sister) fall for him at some point.
At the end of every year he offers an extension with an increased penalty. Failure to take the extension 1st year means sex with protection, failure to take the extension the second year means raw non-cream pie sex and finally failure on the 3rd year means cream pie sex during the girls period (implying that all sex he threatens the girls with normally in non-reproductive). If the other girls he is assigned to teach are as prideful as the four we have been introduced to have been pretty much all of them are guaranteed to have taken the extension. What this new Geas does is give both Tsubaki and Shiren a 2 year extension with the penalty being reproductive sex if Tsubaki wins the graduation day duel.
It is interesting to note that he has both left escape conditions for the girls in the various geas (even if they won't notice them due to pride) and also has multiple ways of fulfilling the penalty conditions applied to himself. If Shiren wins the duel, he only needs to wear a mask and no longer teach in the school (doesn't mean that the principal won't send him troublesome female students to teach wherever he ends up).