Is It Permissible for a Muslim to Be a Martial Arts Trainer?
Question

I do Muay Thai, and I love martial arts in general. I would love to become a trainer when I finish my studies, but if I work as a trainer, I will be required to enter students into competitions. I have read a Fatwa which says that fighting in competitions is not permissible, because it involves striking the face, which is forbidden.
So is it permissible for me to become a trainer, or not? If I become a trainer, and someone comes to train with me, then I tell him that I do not allow students to enter competitions, will I be sinning?
Praise be to Allah.Are martial arts allowed in Islam?
There is nothing wrong with practising martial arts, if they are free of the following matters, which are objectionable according to Islamic teachings:

Doing anything that is prohibited, such as striking the face and head, or causing harm to one’s competitor .

It says in a statement of the Islamic Fiqh Council belonging to the Muslim World League in Makkah Al-Mukarramah regarding the topic of boxing , freestyle wrestling and bullfighting: 
“The Council unanimously agreed that the boxing mentioned, in the way in which it is actually practised in boxing rings and competitions in our country today, is a sport that is prohibited according to Islamic teachings, because it is based on allowing each contestant to cause extreme physical harm to the other, which could result in blindness, acute or chronic damage to the brain, severe fractures, or even death, without any liability on the part of the one who hit him. In addition to that, there is the joy and excitement that the audience who support the winner show at what happened to his opponent of harm. This is a prohibited action which is completely and utterly rejected by Islam, because Allah, may He be Exalted, says (interpretation of the meaning):
{and do not throw [yourselves] with your [own] hands into destruction.} [Al-Baqarah 2:195]
{And do not kill yourselves [or one another]. Indeed, Allah is to you ever Merciful.} [An-Nisa’ 4:29]
Based on that, the Islamic jurists have stated that if someone allowed someone else to shed his blood and said to him: Kill me, it is not permissible for him to kill him, and if he does that, then he is responsible and deserves to be punished.
Based on that, the Council affirms that it is not permissible for this boxing to be described as a sport, and it is not permissible to engage in it, because the concept of sport is based on exercise without causing harm or injury. It should be removed from local sports programs and it should be disallowed to participate in international contests in it. The Council also states that it is not permissible, according to Islamic teachings, to broadcast boxing matches on TV, so that the new generation will not learn this bad practice or try to imitate it.
As for freestyle wrestling, in which each contestant is allowed to cause harm or injury to the other, the Council thinks that it is very similar to boxing, which has been mentioned above, even though its format is different, because all the objectionable matters referred to in the case of boxing are also to be found in the case of freestyle wrestling, which is done in contests, so it come under the same ruling of prohibition.” 

Uncovering of `Awrah. The `Awrah of men is from the navel to the knee.
Mixing between the sexes which includes training girls.
Distraction from remembrance of Allah and causing people to miss prayers and other obligatory duties.
Bowing to other participants or trainers .
Odd and deviant ideas and anything connected to idolatrous religions.
Music , drums and other musical instruments when engaging in this sport .

If the sport is free of these objectionable matters, there is nothing wrong with engaging in it.
Can Muslims practise Muay Thai?
Muay Thai, or Thai kickboxing, includes a number of the objectionable matters mentioned above, such as striking the face, causing harm or injury to one’s opponent, uncovering `Awrah, and using musical instruments.
It is not permissible to engage in this sport unless you avoid these objectionable matters.
Can a Muslim work as a martial arts trainer?
There is nothing wrong with a person working as a martial arts trainer if he can make learners avoid these evils. If he cannot do that, then it is not permissible for him, because in that case his work will come under the heading of cooperating in sin. Allah, may He be Exalted, says (interpretation of the meaning):
{And cooperate in righteousness and piety, but do not cooperate in sin and transgression. And fear Allah; indeed, Allah is severe in penalty.} [Al-Ma’idah 5:2]
And Allah knows best.