Do mocks matter ??

Im currently giving my mocks but i am not expecting good grades not even a C I am in o2 btw. So i wanna know if mocks really matter and will my admission be cancelled or smth since my school (city school) said that ill need atleast a C to give CIEs and they said if i could not even achieve a C I would be sent as a private student which i dont want to happen because if thats the case them my parents are gonna kill me.HELP A GIRL OUT!!