Is it permissible for a group of girls to stay overnight in the mosque (“sleepover”)?

Islam does not stipulate that a woman should have a mahram
with her except in the case of travel. If she wants to go to the mosque or
the marketplace or to visit a neighbour or relative, and the place is nearby
and the one who goes there is not regarded as being a traveller, then there
is nothing wrong with her going there on her own without a mahram. But it is
stipulated that the place should be safe, so the route should be safe with
no danger to her as she comes and goes. 

If the route is safe and the girl can reach the mosque, there
is nothing wrong with her meeting with her sisters to do some communal
actions such as seeking knowledge, or individual actions such as reading
Qur’aan and praying. As for staying overnight in the mosque with her friends
and sisters, it is permissible in principle if the women’s prayer space is
secure and safe from men entering it or from the foolish trying to overstep
the mark with them. That is a matter to be weighed up by the people in
charge of the mosque, namely the administration, the imam and the committee
that looks after it, as well as parents. Every environment has its own
rulings. The most important thing is that the girls should be safe from men
entering upon them and the foolish trying to overstep the mark with them. It
is preferable for there to be someone who can take charge and take care of
them, such as a woman who is a seeker of knowledge (taalibat ‘ilm), so that
no fitnah will result from their gathering and so as to make sure they
behave themselves in accordance with Islamic guidelines and do not waste
their time. 

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have
mercy on him) said: Women may observe i‘tikaaf so long as no fitnah results
from their doing so. If it will result in fitnah, then they should not be
allowed to do that, because if something that is mustahabb will result in
something that is forbidden, it must be prevented, just as if something that
is permitted will result in something that is forbidden, it must be
prevented… 

… But it may be said: How can (a woman) observe i‘tikaaf in a
mosque in which prayers in congregation are not held? Is that not a source
of fitnah? 

The response is that it may or may not be so. This mosque may
be safe and secure, and no one enters it, so there is no fear of fitnah for
the women if they observe i‘tikaaf there. Or it may be the opposite. The
point is that if fitnah will occur, the women should not be allowed to
observe i‘tikaaf, no matter which mosque it is. 

End quote from ash-Sharh al-Mumti‘ Zaad al-Mustaqni‘,
6/510, 511 

It is not permissible for women to raise their voices to the
extent that they may be heard by men. If women are forbidden to say tasbeeh
(“Subhaan Allah”) to alert the imam if he makes a mistake in the prayer, and
they are forbidden to give the adhaan (call to prayer), lead the prayer and
deliver the khutbah (sermon), all of which involve dhikr or remembrance of
Allah, then it is more appropriate that they be forbidden to raise their
voices in other kinds of speech. 

Ibn Hajar (may Allah have mercy on him) Women are forbidden
to say tasbeeh (to alert the imam to a mistake in the prayer) because they
are enjoined to lower their voices in prayer in general, because of the fear
of temptation (fitnah). 

End quote from Fath al-Baari, 3/77. See also the
answers to questions no. 9279
and 39186 

This is as far as the shar‘i ruling is concerned. However we
do not advise staying over in the mosque for the purpose mentioned; we think
that it is sufficient for the girls to spend the day there, whether that is
studying together or worshipping together, then they should go back to their
homes at night. 

We ask Allah to protect them, take care of them and help them
to do that which pleases Him. For more information please see the answer to
questions no. 49898 and
26298 

And Allah knows best.