age rating - week 6

I have researched on age ratings and i have decided with my group that our opening sequence is going to be at a age of 15. we have a choice between 12A, 12 and 15. We didn't pick 12A or 12 because then we cannot use any violence whereas our filming has some violent scenes where someone is getting stabbed with a knife and there is blood everywhere. I had thought that 15 is a good age rate as the teens are more mature then and are able to handle films with knifes and blood.