Checking phones in lectures can cost students half a grade in exams. Students perform less well in end-of-term exams if they are allowed access to an electronic device, such as a phone or tablet, for non-academic purposes in lectures, a new study. Source 5h.
This is probably for the best. Multitasking has been shown to cause performance in both activities to suffer. This is why taking phone calls in the car, even when hands free, has been banned in many states.