can a cop just open my car?

[Cruz]

alright so today i go to pick up my friend from school cause he only has school twice a week until eleven. my girlfriend who goes to that school comes out to see me also when were about to leave the cop comes out of the school and comes running over to my car and opens my passenger seat and starts accusing us of smoking cigarettes which i did earlier but not at the school. so i come out with it and say yes i did smoke a cigarette cause i've learned lying to cops is a bad idea... than the k9 dude comes out and searches my car and finds an old bag of weed that i totally forgot about.. my charges was possession of mary jane, a minor tobacco, and paraphernalia... now my question is can the school cop come up to us and open my car and start yelling at us for something he had no Proff of... i understand that i was on school property but can they really just open my car with out permission don't they at least have to knock and ask to get into my car...