Why does my iPhone use excessive amounts of data?

My dad, step-mom, brother, and I are all on a 40gb data plan through AT&T. So that gives us each 10gb to use throughout our billing cycle each month. Over time my phone has gradually started to use more and more data. I was on a 2gb data plan when I had an android. After I got my iPhone i kept the same plan but a few months later I kept going over my data so I got a 4gb data plan. I then began to go over my 4gb data plan so I thought maybe it was just my iPhone. So I got a iPhone 5s.. It stayed below the 4gb plan until around 6 months ago. It went from 4gb to 6gb to 8gb over time and around January I was going over 10gb. February my phone was using 10gb in 2 weeks. This month it used 10gb in 12 days. I am on wifi at school so from 8:30-3:30 and I'm on wifi at home, I usually get there around 5:30 or 6. So, I contacted AT&T. They said I must me streaming videos and music constantl, but I'm not. I have turned off data to my apps. I have downloaded a data manager. I turned off my location services and the setting to automatically load videos. When I'm on wifi I turn my cellular data off. I kill my apps as soon as I'm done using them. I have even turned off setting for my apps and iCloud. I just don't know what else to do.