Is it permissible for a committed young man studying in high school, who is taught strange subjects (like music), to leave his studies and devote himself to memorizing the Qur’an in a mosque, or should he remain in the mixed-gender school?

It is essential to memorize the Quran and study Islamic knowledge, and it is obligatory to stay away from places of intermingling. If it is possible to combine memorizing the Quran, studying Islamic knowledge, and studying curricula to obtain a university degree while avoiding forbidden things, then that is more befitting. The study of subjects that include forbidden elements, such as music, should be abandoned, and one should focus on other subjects to compensate for the deficiency. If you are compelled to know about them, then take what students have written about them to pass. If you are not in need of studying and you have the capability to fulfill the communal obligation (fard kifaya) of studying Islamic knowledge, then dedicating yourself to that is more befitting.