I would never leave my guitar in my car and certainly not in the trunk for a long period of time. The extreme heat or cold and going from one extreme temperature or humidity to another will damage the guitar finish. It can crack the wood and warp the neck. It may not bother a solid body electric as much as an acoustic. I take my guitar to work with me almost every day. Some of the guys and I jam during our lunch break, but I always bring my guitar into work with me. I never leave it in the car. There must be some where in the building that you can safely store your guitar while you are working.
If I want to make a stop at a store on the way home from work, I take my guitar home first or take it in the store with me. I take my guitar into Guitar Center and check it at the counter. I take it in the barber shop, drug store, etc. Never leave it in your car!
Do what you want, but I'm sure your guitar instructor will tell you the same.