I have a betta. It's pretty easy to tell when he's feeling something; if you've ever had or seen one, you know they're pretty expressive little guys. Granted, he mostly seems to be anticipating being fed (or the possibility of lady bettas wandering by the nests of bubbles he builds), or getting pissed off (which he displays by war-dancing -- just youtube it) at the snail living in his tank, but he's fully capable of interacting with humans at the level of most other sorts of pets. I'm not anthropomorphizing. If he could breathe air, he'd be doing his very best macho impersonation of a tomcat (or small yippy dog that thinks it's approximately the size of a great dane).
If we go one step up and think about the average high schooler, I still am having trouble differentiating what (the high schooler) he or she might feel from what the betta might -- hunger, sexual frustration, happiness, annoyance, loneliness... the betta probably can't do trigonometry, doesn't have opposable thumbs, and has a different set of instincts tailored to his environment. We've simply evolved to do different things; that doesn't mean humans cannot observe emotions analogous to our own in other creatures.
Spider-wise, I've never observed one for periods of time long enough to do much more than remove it from my house, but who knows.