Is it permissible for me to become a martial arts trainer—specifically Muay Thai—knowing that this will require me to enter trainees into fighting competitions that involve striking the face, which is Islamically forbidden, and would I be sinful if I stipulated to the trainees that they not enter competitions?

Martial arts are permissible if they are free from Sharia (definition) prohibitions, such as harming the opponent, uncovering 'awraat (private parts), mixing of sexes, distracting from the remembrance of Allah, bowing to other than Allah, and the presence of deviant ideologies and musical instruments. Accordingly, Muay Thai is not permissible unless these prohibitions are avoided. Working as a martial arts trainer is permissible if it is possible to keep the players away from forbidden acts; otherwise, it is not permissible, due to the Almighty’s saying: (And cooperate in righteousness and piety, but do not cooperate in sin and aggression).