Do most people (especially students) in Montreal know how to speak English?

In Montreal, you will not have a problem speaking English. Most people who work in tourist-related industries (hotels, restaurants, stores) speak English or make the attempt to do so. Don't expect to be addressed in English when you enter these establishments.

There are two internationally well-known English universities in Montreal, Concordia and McGill. So, there are plenty of students around the city who speak English.

If you begin speaking in French and then find you have trouble with the conversation, someone in the conversation will likely help out with French or may interject some English, or you could use English for things you're not sure about. That isn't a crime.

Well, Probably they know at least some English, It is a main language and most parts of the world speak it. Canada is both English and french, but because it has both as main languages they'd probably know, or speak it.

Montreal is a big tourist city.

You'd be surprised how many people speak English.

If you know basic french, you'll be perfectly fine.

yes, there are two big english universities and a lot of tourists so there is no problem meeting people who speak english.

if you can speak french "decently well" you're pretty much set.

Yep. I'm a student. I speak english.