Will the battery on my iPhone 4 last this long?

Okay so tomorrow is my first day of highschool. (Obviously I'm gonna bring my phone, maybe check it occasionally in the halls from class to class. I won't be texting in class.) And I was wondering if my battery would last as long as I need it to. It's an iPhone 4, and I'm charging it all night so that it's at 100% tomorrow. In the morning I have to take a 20 or 30 bus ride, which I'll probably be on my phone for about half of, then I'm in school for 7 and a half hours, then another half hour bus ride home. So will my phone last about eight hours, if I'm only checking it occasionally, and using it a bit on the bus rides? I guess if it won't last I could always bring a portable charger just in case. Thanks in advance.

P.S. Please don't give me an answer like "you're there to learn, leave your phone at home. I bring my phone everywhere with me, as I have very bad anixety, and even if I'm not constantly using it, it makes me feel better knowing that I have it with me (incase of emergencies.)

Yeah, sure it will last… When i had iPhone 4 i used to play games, watching videos for like 4 hours and still had like 30%-35% left but if you only use it occasionally then it will last for sure.