You are missing the point! You must allow the girl to marry and the married to have their babies. It just requires good management in the yashivas. The public schools in America have no problem with this issues because they use substitute teachers. The reasonable way to do it
1) Have the teachers sign contract for the year to teach. They can get married or have babies within the contract. New mother can get the time they need with substitute teachers. The new wives will have to fulfill the contract for the year after they are married.
2) BUT, the yeshiva has to fulfill their side of the contract, like paying them on time. If not, they have the legal and halachic right to consider this a braking of the contract and can resign.