Should a principal enforce the rules in his school, YES! Definitely 100%. However, should he go around asking bochurim to empty their pockets and/or bags to search their belongings for cellphones, then definitely NO! It is embarrassing, counterproductive to a high school mentality (most teenagers would feel that they are being picked on and will be embarrassed if it is done in a public area), and it is 100% ILLEGAL to do in the U.S.A. It is a violation of ones personal rights.
If the boy is openly using in the school then the Principal, abiding by the rules of the school, should confiscate it and return it at the end of the day or send it back to the parents. However, the principal should not actively search for them when they are put away. And the school should not have enact rules where it is illegal or extremely hard to enforce.