This might seem odd, but the best portable whiteboard I've ever used is not a whiteboard. It's a Chromebook cover that doubles as a whiteboard. The Cranium is made of PVC and aluminum, and the white surface is better than any other cheap, particle-board whiteboard you've ever used. You can stick it on a Chromebook, or you can just carry it around. Stick it on a binder. Maybe the side of your car. Slap one on the wall in your garage. Keep the tape on and wait until it fits the right Chromebook, and slap it on that.
Seriously, you will notice the difference between this and other whiteboards. It erases clean and bright white. It's solid and waterproof. Dry erase or wet erase. Yes, it can double as the best Chromebook protector out there, but this is really the best portable whiteboard out there, so buy it as such. Get a classroom set that will never need replacing. Issue one to every student. It's even better if you have Chromebooks to stick them on. But if your school district can't swing the price tag for 1:1 computers, then get these awesome whiteboards while you do some fundraising.