2. I have a friend who's on a visitor visa in the U.S. If he gets an admission to a school here and gets an I-20, would he be able to change his status to a student?
It is possible to apply to change status from tourist to student. Whether it is approved or not depends on several factors including how long your friend was here before applying for the change of status, what the friend told the consular officer when he applied for the visitor's visa in the first place, and whether he has adequate financial support to go to school. An additional factor which is sometimes raised by INS, but more often by consulates, whether or not he has shown that he intends to return to his country.
Please note that if he travels abroad after the change of status approval, he would need to obtain the F-1 visa at a consulate.