Short answer
Before puberty, mixed classes are not a problem in themselves. From puberty, Hanafi scholars do not permit free mixing of boys and girls, so a single-sex school should be chosen where one is available and suitable. Where there is no real alternative, a mixed school is tolerated out of need, with care: modest dress, lowered gaze and no dating or seclusion.
Free mixing between non-mahram adolescents and adults is not permitted, and seclusion is haram. Deobandi fatwas therefore treat co-education after puberty as impermissible where it brings free mixing, and tolerate it only out of real need, with the rules of hijab, gaze and conduct kept.
The four schools agree on this.
Explanation
Allah tells believing men and women to lower their gaze and guard their modesty, and the Prophet ﷺ warned against men entering upon women in private. These rules apply from puberty. Young children may learn, play and sit together without harm. The four schools agree on the principles; scholars today differ on how strictly to apply them to schools in non-Muslim countries.
Primary school: mixed classes are fine. Focus on the curriculum, the values taught and the friends your child makes.
Secondary school and college:
- A good single-sex school, Islamic or not, is the better choice where you can get a place and afford it.
- If the only realistic option is mixed, it is tolerated out of need. Teach your child the reasons behind modesty, not just rules.
- Keep relationships to classwork and group projects. Avoid one-to-one private meetings, dating and online chatting that slides into intimacy.
- Girls keep hijab and modest dress; boys dress modestly too.
- Stay close: know their friends, their phones and their worries.
University and adult education follow the same principles: seek the needed knowledge, keep the limits of conduct, and choose settings that help you keep them. Islam values education for both sexes; see education for women.
For other school choices see homeschooling and Catholic schools.
The evidence
Quran, Surah An-Noor 24:30
قُل لِّلْمُؤْمِنِينَ يَغُضُّوا۟ مِنْ أَبْصَٰرِهِمْ وَيَحْفَظُوا۟ فُرُوجَهُمْ ۚ ذَٰلِكَ أَزْكَىٰ لَهُمْ ۗ إِنَّ ٱللَّهَ خَبِيرٌۢ بِمَا يَصْنَعُونَ ﴿٣٠﴾
Tell the believing men to reduce [some] of their vision and guard their private parts. That is purer for them. Indeed, Allah is Acquainted with what they do.
The command to lower the gaze, which applies at school too.
Sahih al-Bukhari 5232
Sahih... إياكم والدخول على النساء ...
"Beware of entering upon women."
Warns against private mixing between non-mahram men and women.
Where the ruling is recorded
- Radd al-Muhtar (Ibn Abidin), Kitab al-Hazr wal-Ibaha, section on looking and touching. Rules on gaze, seclusion and mixing with non-mahram people.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.