The town of Montclair is mostly older, wealthy people, so there aren't a ton of clubs here (plus, Montclair is not really a nightclub’s top location choice when they can just set up shop in the city, which is so close). Also, Montclair is not a college town, so there aren’t a lot of bars within walking distance of campus. You can always go into New York City or Newark, but public transportation stops running late at night, so you could get stuck there or end up paying a lot for a taxi ride back. The nearby clubs and bars are not super cheap, but there are a lot of places that offer drink specials; the website Awesomenj.com offers a pretty extensive list of drink specials in the area.
There are not a lot of house parties, or if there are, they aren't easy to randomly come across, since most student-owned houses are not nearby. There are parties in the dorms, and those are a bit easier to find. One reason the nightlife is so lackluster at Montclair is that the majority of the student body doesn't live on campus. Those students go home, or anywhere really, to party—they don't need bars or clubs to be close, so there aren't too many nearby.