How can you tell if someone is planning on commiting suicide?

Them becoming either distant or angry if you ask them certain things, the way they talk eg. saying the world would be better with out them around.

Ask them — seriously.
Do they have a plan? Do they have access to resources that could carry out their plan?
If they have been severally depressed and then suddenly seem to improve this could be a sign they have decided to attempt suicide.

Iwould say c. take responsibility for your behavior

The administration should take action.