I want to firmly reject the invitation of friends to not go to school and smoking. But I was afraid would happen rough. What is roughly the same firm?

My brother is a good heart,

Solidify but do not be rude.
...
Raises respectful firmness.

Assertiveness that you convey that caring for the good of others, and you convey a loving way, will actually bring them to you.

Do not be rude.

Roughness and hardness invite retaliation.

Remember, revenge is rarely announced when he will come. And usually he came up with the ugliness that is greater than that caused it.

Be faithful to the true, and edify each other in patience.

With it, you will become a beautiful person.