In the Name of Allah, most Compassionate, most Merciful

It is known from the following ayah and hadith that looking at someone of opposite gender (who is not close relative) is haram in Islam.  Just talking with them or looking at them mounts for the adultery of the eyes and tongue!

Quran 24:30-31. "Tell the believing men to lower their gaze, and protect their private parts. That is purer for them. Verily, Allah is All-Aware of what they do. And tell the believing women to lower their gaze, and protect their private parts ..."

Quran 17:32 "And come not near to adultery (anything that leads to adultery). Verily, it is a Faahishah (i.e. anything that transgresses its limits: a great sin, and an evil way)."

Going to mixed school is a disobedience to Allah because the atmosphere of mixed learning causes looking at others.  It is a way of going close to committing Faahishah.  Allah ordered not to go close to things like that.  So Allah is banning mixed schools and any mixed gatherings!  Allah said that hearts would become impure if they do it.  The commitment to the religion will degrade or will be wiped out.  The reward in hereafter will lessen or wiped out.

Bukhaari, 5889; Muslim, 2657 by Abu Hurayrah
Prophet (peace and blessings of Allaah be upon him) said: Allaah has decreed for every son of Adam his share of zina, which he will inevitably commit. The zina of the eyes is looking, the zina of the tongue is speaking, one may wish and desire, and the private parts confirm that or deny it.

The parents who send their kids into mixed schools not only don't shun the evil required from Islam but also raise their kids into the adultery atmosphere.  Not shunning the evil but promoting it (making it halal) and sending the kids into virtual prostitution surmounts to leaving the folds of Islam.  The people who go to mixed schools for a bright future or good outcome are only going for their destruction and away from Allah's blessing and help.

As Prophet Yusuf (peace be upon him) knew all this and chose to go to jail and waste his life in jail then commit adultery knowing how big a sin it is!  It was from Allah that after many years he was pulled out of the jail.  So should we be concerned about our kids' future and our future in this world first or the the future of hereafter!  Prophet Yusuf knew that the jail and the torment in this world is nothing compared to the torment and jail of hellfire!

The above also apply for watching TV where a person can possibly look at opposite gender.