I would actually recommend any student to start working as professionally as possible, as early as possible. I would for CAD actually never recommend working on a laptop but on an external flatscreen monitor. It’s just so much nicer and you can get them at 100$ if you’re lucky. So if you have a locker at your college then you can just put it in there and get it out when you need to put in some work hours.
I don’t know how long you’ve been studying already and how strong your back is, but take into account the amount of time and during which activities you’re going to be carrying around the thing. I would rather cut off a few pounds than end up with lower back pains (i learned that the hard way). If you’re going to have to bike for 20 minutes+ every day with a lot of weight in your backpack, I would probably go for a lighter one. Many of my colleague designers/design teachers seem to be going along with a local 13" Macbook Air trend/epidemic, although they don’t use it much for CAD so I don’t know about doing CAD on a 13" screen. In my experience, compared to a 24" monitor also a 15.6 inch laptop screen is just a bit too small.