Please help..

How do I get my hair to stay straight? I see other girls at school and everything and their hair says really straight all day but when I straighten my hair it gets "out of place" and like wavy during the day. I often sneak a brush to school and brush my hair back into place multiple times. Is there like a spray (I know about hairspray) but like something to help my hair look nice and straight when I straighten it ? Thanks and sorry this isn't for this app I don't know who else to ask.