"refectory" is not a word you hear commonly, I didn't know what it was until just now so if you are new to English and don't need to use it, I think there are more important words to learn. "refectory" is specifically for students. "canteen" is like a small food shop and is commonly used in schools but also in some parks or recreational areas where you can buy a snack. (the same word could also mean a bottle for holding water) "dining hall" is usually a larger place and includes the area where you can sit down to eat. This could be related to schools but is often used outside of school as well. Different parts of the word seem to use a different word for the place where you get food at school but they all mean the same or similar thing. The most common in Australia that I've seen are "kiosk", "canteen" and my favorite "tuck shop" which comes from an Australian slang for Food "Tucker". There are many words for this but let's be realistic, I don't know them all and why should you. Most places will use one word to refer to their schools lunch shop so just learn one now (I'd recommend "canteen") and if you need another one later then learn that one later.