If you keep your laptop open during class, you're not just distracting yourself, you're distracting everyone behind you (that's how human attention works - if you see a bright display with moving things, your attention is drawn towards it), and that's not right. That's why at my uni, there was an unspoken (de-facto) policy that if you keep your laptop open during lectures, you're sitting in the backrows, especially if you play games or do stuff like that. It worked great - I was always in the front row with pen & paper.
However, a laptop is very useful to get work done during breaks or labs when you're actually supposed to use it.