Sorry - late to the game
I would say it really depends on your current habits. There is some peer pressure to constantly go out the first month - definitely. But after that everyone settles in and does not feel the need to show off. Also the first set of tests usually comes around then, so sanity returns. Some people do go out more while in business school but you will meet people from all walks of life and can assume any pattern you want (hand out at friends' homes or in uber martini bars). There are party guys - a large percentage in most schools. They may be used to fancy restaurants and expense accounts - I could never keep up, so I never tried. My preference was always to spend money on food rather than alcohol, but that was probably because I was poor.
I would say, there are some "required" functions such as mixers/networking events/others that you will need to go to, but they are just 1-2 per month, so that would be your rock bottom minimum (include a drink or two and valet parking).
This kind of warrants a bigger discussion about the MBA budget in general, which I will slightly touch on:
I think my biggest surprise was the cost of textbooks - anywhere from $200 to $500 per quarter (double that per semester). I shopped on Amazon and other places and strategically sold my books. Not sure if that's the case in all Bschools, but definitely can be a burden. That messed my budget up pretty badly.
Don't forget about medical insurance - your school may provide a plan, so check about that.
I spent little on clothes - just got the seasonal items I did not have, and relied mostly on my pre-MBA wardrobe. As I did not work out much during school, I gained about 10-15 lbs over the two years, so that took a toll. I still remember as I was trying to get a pair of my nice slacks for a vegas trip and they would no longer fit me.
I structured my budget in terms of priorities and so my entertainment expenses were at the very bottom of that list. I under budgeted and as the result, my first semester was very boring - that's how I ended up starting GMAT Club