Can an adult man take pictures of teenage girls without their knowledge?

There is nothing illegal about simply taking photographs of teenage girls.  however, the fact that the man is taking the pictures, espcially if it is of the same person on more than on occasion, could rise to the level of stalking, which is a crime and may expose the person to jail time.  Based on today's politics regarding older men taking pictures of young girls, the man can be exposed to prosecution under the stalking statute or harassment statute as there does not appear to be any justification for taking the pictures, especially using a cell phone, that can be said to be anything other than related to sex.  I suggest that the man stop taking the pictures as he may expose himself to criminal liability should someone notice.  this is not something that the person needs to have happen, as it will be publically embarrassing.