Lasombra refusing to give Rhonda their WiFi password is portrayed as him just being a dick. Until you realize that she can use the WiFi to call for help and that Lasombra would surely be aware of this.
It may seem odd that Ruth would be at Arnold's party (no weirder than Lockjaw, though) since she doesn't really know him all that well and he's never helped her directly. They only had one conversation, and she has no idea he had a crush on her. So why is she there? She does know him, since she's smiled at him a few times, and a handful of Arnold's good deeds affected people beyond his class. She went to his "Geek Party" and is likely aware that his ideas ended the teacher's strike, something that would definitely affect her!
Some of Lasombra's men are actually heard moaning in pain after the spike trap nails them. We also don't know for sure if the mook who fell through the pitfall is toast or not.
So if Arnold and company are fifth-graders, did they participate as the perpetrators of "The Longest Monday"? The day where fifth graders stuff all the fourth graders into the trash cans? Harold, Sid, and Stinky seem like they would, if no one else.
Actually, there are some points showing that Sid is the more moral of the three of them, such as when he covered up their mooning during Arnold's contest entry video. On top of that, maybe they (the whole class) decided to get rid of "Longest Monday" since they know how it feels to be on the receiving end.
At the end of that episode, Arnold and Gerald admitted that getting canned wasn't that bad and consider continuing the tradition (Gerald especially seemed eager to do it after they got mocked by those third graders).
Wouldn't it have easier to try and guess the wifi password and ask for help that way rather than smashing Rhonda's phone? They even stated that it was written down somewhere.
It's possible that Lasombra kept a copy of the password with him so that no one would be able to send a message for help. He just underestimated them. On top of that, Rhonda guessed the initial password by accident. It would take quite some time to guess the new password.