Does everyone who lands an internship with one of the Big4 get an offer for a graduate scheme?
So if you have managed to secure an internship, you have done the most difficult part!
To answer your question - the vast majority of people that do the internship are offered a place on the graduate scheme. They are not expecting you to be the perfect graduate as yet (most students have a year left at university after all), however they are expecting you to work very hard, be eager to learn/ curious about the role and the firm, network, be on time, be professional and act like you want the job! Do these things and you will most probably be offered a grad role.