Is it permissible for the head of a religious school to build a congregational mosque or any other structure on land endowed for the school, given that most of the endowers and its specific superintendent have passed away? And what is the ruling on Iʿtikāf and Tahiyyat al-Masjid therein according to the Shafiʿi madhhab, with textual evidence?

The supervision of an endowment (waqf) falls to the person stipulated by the endower (waqif). If the endower did not stipulate anyone for supervision, or if the person stipulated dies, then the supervision belongs to the legitimate ruler (al-hakim al-shar'i). In the absence of a legitimate judge, the Muslim community (jama'at al-muslimeen) is to appoint a just, sane, adult Muslim to undertake the supervision of the endowment and act according to the endower's condition.

The endower's conditions must be observed and acted upon. There is no harm in deviating from them if acting upon them is impossible or would cause harm to the endowment, such as when the condition contradicts Islamic law or leads to the endowment becoming inactive.

The just supervisor must consider the endower's condition. If building a mosque does not contradict the condition, then the mosque should be built. Or, if it contradicts the condition but the public interest necessitates its construction, then it should be built, and the rituals of the religion should be performed therein.