I was there 2 years ago during Grad Night week and the only impact I noticed was the park closure time (it was around 8 if I remember correctly) and certain areas of the park being roped off and prepared while we were still there. For example, as you're heading towards the castle, off of the left and right they were roping off the area and setting up the area for the dj's, dancing, etc. and they were directing traffic away from that part of the castle. It wasn't a big deal because of all the other ways to go around that area. Other than that, I didn't notice any difference crowd-wise and nancy drew--don't avoid visiting then, it shouldn't be a problem.
Sidenote because whenever I think about this I get so mad--when I was in high school, every single graduating class BUT ours got a
Disneyland Grad Night. Our principal thought he was doing us a favor, saying, "Why would you want to go to a little kid place anyway???".

To this day, 18 years later, I still seethe when I think about that. My mom, dad, brother...my whole family really (aunts, uncles, etc.) had a Grad Night at Disneyland. Anyway, sorry to go off on a tangent, I just had to release the steam.