Re: oral cancer
I'd listen to your dentist and get it checked out. Did you tell your dentist that you smoked and did you ask him if it could be from smoking?
It is very important, if you smoke, to tell your doctor that you do. If you lie, you could really hurt yourself. Also, if you're not mature enough to be able to tell your doctor that you smoke, then you're not mature enough to smoke.
The white mark could be from smoking. Although you are young, people are affected differently by tobacco smoke and so you might be more sensitive than others, causing you to experience its negative affects earlier in life.