Each lecture you go to, you receive one ingredient; each tutorial, up to three ingredients but you have to sauce-fight your tute mates to get the best ones. Bread is "unlimited" in that you can collect it as many times as you like, but to get to the breadstore you need to make your way through the breadknife labyrinth and climb Sandwich Hill, and even then you can only take as much bread as you can fit in your toastrack (distributed at the beginning of the year, and usually worn on a chain around each student's neck to avoid theft. That said, if you want to focus on fatties - fat sandwiches (with many ingredients) - rather than more numerous simple sandwiches, you can sell your toastrack for a handsome fee to someone who wants a carrying capacity of up to 16 slices, and you can just collect a small pile of bread in your hands when you visit Sandwich Hill. You just have to be careful you don't drop it when you climb over the Burning Toaster Stile to get back to campus). Obviously there are different bread varieties on the Hill.
Anyway about grading, once you have enough ingredients to make a good sandwich, you simply construct it and attend the daily lunch ceremony in Breadbin Hall at the centre of town. You present your sandwich to the Sandwich Master of your choice and they will award you a gold, silver or bronze pretzel token (or nothing at all) depending on various set criteria and their own personal taste. At the end of the year the person with the most pretzel points wins!
It's been a while since I did my sandwich course at university, but if you have any questions just ask!
Edit: As SnoochtotheBooch rightly pointed out, one human year is approx 100 sandwich years; as a basic guideline you should be aiming for one sandwich every sandwich year but some people make a lot more.