What is the ruling of the Shari'ah on one of the teachers remaining with the Quran memorization students at the back of the mosque during 'Isha prayer, which might cause him to miss some rak'ahs in order to keep them in order?

Bringing up boys to be good and guiding them in matters of Islamic law is a virtuous act and a praiseworthy objective in Sharia (definition). Straightening the rows is recommended in Sharia according to the majority of jurists, while some consider it obligatory. Therefore, there is no harm in someone who straightens their rows being late, especially if they catch a part of the congregational prayer.