How to deal with a friend who cuts
Although popular culture and public opinion may lead people to believe that self injury is either a phase or people seeking attention. The truth is that this behavior becomes an unhealthy coping mechanism for any number of unpleasant scenarios.

The idea of distractionary coping mechanisms by themselves are not harmful, but self injury is specifically more dangerous because causing injury in the first place opens ones self up to infection, permanent physical damage, even accidental suicide.

Self injury often co-occurs with another mental illness, both disorders need to be diagnosed and treated by a decent psychiatric team which may include medications, but most definitely therapy. This is much more than a school therapist can provide. There is more information here: Mayo Clinic on Self Injury

If your friend is not receiving the level of help that he needs or he cannot commit to safety, it would be best to take some type of action.

Possibly:


Call a help line
Inform the school therapist 
If your friend has any supportive family members, maybe informing them would help
In any immediate emergency call 911