What is the ruling on distributing commercial advertisements for a private school inside mosques by placing them on shelves designated for Qurans, or on bulletin boards, or next to doors? And what is advised for someone who finds them to do?

It is not permissible to use the mosque for displaying commercial advertisements for schools or institutions, because mosques are designated for the worship of Allah through prayer, remembrance, seeking knowledge, and reciting the Qur'an. Exploiting them by placing advertisements inside them is not permissible, and the mosque should be kept free from such activities, as Allah Almighty says: (And verily the mosques are for Allah, so invoke not anyone along with Allah) [Al-Jinn: 18]. And due to what was narrated from Abu Hurairah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: (If you see someone buying or selling in the mosque, then say: May Allah not profit your trade). Displaying these advertisements falls under commerce.