Something assertive, as in
Of the nature of, or characterized by, assertion; declaratory, affirmative; positive, dogmatic
It's true 'assertive' does not denote 'rebellious' but someone who is assertive is not wont to 'conform to rules or conventions' just because they are there.
You could use self-assertive, but the self part is provided by I.
I do like
insisting on one's own rights or claims; egoistic; assuming.
Most teens are wrapped up in their own world, so this seems appropriate. Someone who asserts themselves onto the world is not likely to be prone to conformity.
Or, you could forego the use of any adjective. First adjectives tend to weaken writing because they suggest nouns cannot do their job on their own. Second, isn't every teen one who 'dislike[s] following imposed rules, and [who] want[s] to follow [his/her/their] own path.'?
Definitions from the Oxford English Dictionary, online, available through susbcription.