Whats a good efficient way to organise data?

I think I would start by first defining what the vegetarian nutritional requirements are: things like proteins (beans, cheese), fibre, etc. Its often useful to make definitions like these before you decide how to capture your data as it can expose things you may not have thought of.

Your teacher is right, its a great idea to catalogue all the meals offered chronologically. Then you’ve just got to define your fields: the date, the name of the meal and then a series of boolean (yes or no) fields that indicate if each of the nutritional requirements was satisfied. So for example your column headers might read:
Date, Meal Name, Is Vegetarian, Has Protein, Has Fibre

And a sample set of data might look like:
2008–02-19, Gruel Flambé, Y, Y, N
2008–02-20, Long-Pig au Gratin, N, N, N

With a set of data like this you can show simple but effective things. Off the top of my head I’m guessing you’ll want to take all the meals in a given date range and show how many of them were vegetarian-compatible. Not only could you show this but you could also show if there is a particular deficit of a certain kind of nutrition.

You could even expand on this to include non-vegetarian nutrition requirements and make sure your omnivorous/carnivorous friends are getting what they need too. Further, you could track the cost of each meal and draw correlations between all sorts of things. You, my friend, could start a totally non-violent lunch-menu revolution…with data! Huzzah!