What is the legal ruling on his project, and what is your advice to him, knowing that he is an outstanding student in a co-educational high school who loves a girl there and wishes to marry her, and is thinking of sending her messages to guide her religiously and morally, especially given the presence of reprehensible acts in the other high school for boys?

The default ruling for mixed-gender education is prohibition, except in cases of necessity or need, and then with conditions. It is permissible to marry a religious and well-mannered girl. As for telling a girl you love her, that is not permissible. One must be wary of the footsteps of Satan, which can lead to imagining forbidden things. Advising and teaching a woman about religion is best done by her female peers to avoid temptation, especially when there are mutual feelings involved.