If it got
near the opening it is possible, but its about a one in a million chance.
When you have sexual intercourse, and the male ejaculates, he "shoots" the semen up towards your cervix. That is the best way for the sperm to get where they need to go for you to become pregnant. Getting
near the opening of the vagina doesnt even guarantee they are going
in the vagina.
You are 15 so i'm assuming you dont want to get pregnant, so I suggest that you dont have sex. If you do, please please PLEASE use some kind of birth control. And if you are in a situation where you think you may become pregnant (like the one you have described, sperm getting near the vagina) use your best judgement. If you dont think its a good idea, its probably not.
