I play lineback and im almost 14 and im about the same exact size as you. 5'9 and around 140 if ur fast and you have good hands and your still growing i wud say safety or corner. lets say you stop growing. then corner or linebacker. if u have alot of muscle on you i wud say lineback but otherwise yeah i would stay on safety or corner

you're a better fit at safety. dont lift every day though - that's actually bad for you.

First of all DO NOT LIFT WEIGHTS EVERY DAY. You have to work out every other day to keep your muscles healthy, but you can jog, walk, and run everyday.

second of all your coach is an idiot, there is no way you can get that much muscle in 2 1/2 months.
also 5'9' and 140 is the perfect size for a Safety, Linebackers have to be tall and heavy but some safeties do have a linebacker frame which is good.

You should definetly be a safety