Nor may the dead be washed in a mosque. If changes are introduced into a mosque that harm those praying there, whatever harms them must be removed, and measures must be taken to serve their interests, either by restoring it to its original condition or by improving upon it. And Allah knows best.

He was asked:

May someone who teaches children in a mosque spend the night there?

He replied:

Praise be to Allah. The mosque must be protected from anything that harms it or disturbs those praying in it, including children raising their voices, soiling its mats, and similar conduct. This is especially so during prayer, for such conduct is among the gravest wrongs.

As for spending the night there, if this is to meet a need—as in the case of a stranger who has no family, a poor stranger who has no home, or someone in similar circumstances—there is no harm, provided he stays there only as long as needed and then moves elsewhere. But taking it as a regular place to sleep at night and rest at midday is not permissible.