C - i++ not working inside a while loop : So, I'm helping a friend with her programing class, and we've come across something funny. I have the next code:

    void ingresardatos(struct alumno *lista){
	int i=0;
	char continuar='s';
	while(continuar=='s' && i<20){
		printf("Valor de i al iniciar: %d  \n", i);
		printf("Introduzca el nombre del alumno:\n");
		scanf("%s",&(lista[i].nombre));
		printf("Introduzca la matricula:\n");
		scanf("%s",&lista[i].matricula);
		printf("Introduzca la primera calificacion:\n");
		scanf("%f",&lista[i].calf1);
		printf("Introduzca la segunda calificacion:\n");
		scanf("%f",&lista[i].calf2);
		printf("Introduzca la tercera caificacion:\n");
		scanf("%f",&lista[i].calf3);
	
		lista[i].prom=(lista[i].calf1+lista[i].calf2+lista[i].calf3)/3;
	
		if(lista[i].prom<=5.9){
		   strcpy(lista[i].nota,"NA");
	    }
		else if(lista[i].prom>=6 && lista[i].prom<=7.3){
		   strcpy(lista[i].nota,"S");
		}
		else if(lista[i].prom>=7.4 && lista[i].prom<=8.6){
		  strcpy(lista[i].nota,"B");
		}
		else if(lista[i].prom>=8.7 && lista[i].prom<=10){
		   strcpy(lista[i].nota,"MB");
		}
		printf("Valor de i antes: %d \n", i);
		i++;
		printf("Valor de i después: %d \n", i);
		printf("¿Desea continuar? (S/N)");
		scanf("%s",&continuar);
	}
    }

It's supposed to be a grade list for a class of 20; you introduce the student's data until you press "n", and it saves them on a list. Now, I figured out the pointer part (I just really know Java, so it's kinda weird working on C), but what I can't figure out is how to make the i++ part work. If you run it like it is, it'll start with i=0 on the first pass, then go through all the code, and finally do i++ before asking if you want to continue (it prints it on the screen). But then, when you press "s" to indicate you want to continue, it'll start with i=0 again, and for the life of me I can't figure out why. I tried i++, ++i, i=i+1, and so on, but nothing seems to work. I even tried making i a pointer, but Windows didn't like it and crashed my program everytime I ran it.

If someone could explain just why it isn't working, I'd be eternaly greatful. I've been at it for days now, and I can't figure it out.