My iphone battery runs out very quick?

I only had it for less then a year and my iphones battery runs out very quickly. And it sometimes won charge it too 100 just to 98 and i leave my phone in my locker at school and i don't use it but when i come back it drops all the way to 70 or 56 and once it gets to 80 it just drops very fast. I tried doing the battery save thing and i don't know

Try turning off your brightness and doing multiple things can lower your battery quickly, like listening to music and texting/doing things on another app.

It can save some power if you double click the home button then delete all the apps from the little bar that opens