haha im in a pretty simalar situation. im 13 going onto 14 in 2 weeks or so. my parents found my pipe(i left it in their room when i was stoned looking for a lighter) and said dont smoke for 2 years so i thought they smoke so i should be able to some times so i smoke once in a blue moon.

and about telling your parents make sure you do this:
- grades up

- dont spend all of you money on it especilly if you are getting you money form them, because if the're against it, there gonna cut you off(my parents stopped giving lunch money cos they "have no money" i wouldent be suprised if they hid the cough medicine)

- Say you smoke out of you own free will and you wernt pressured or they will stop you from seeing your friends.

- DONT STONE ON SCHOOL DAYS they will not like this at all

- and be honest as possable remember theres the truth and the "truth"

- leave a smell in you room if you cant get the courage and wait for them to talk

why do you want to tell them anyways?