I don’t think you would be allowed to openly evangelize on most high school campuses and for obvious reasons. How would you feel if some Mormons or Hari Krishnas had set up a table at your child’s lunch room and were trying to convert kids?
However, you might be successful setting up a group for Catholic students at the school. If it is a student-led initiative, they may be able to form a student organization. Such an organizaiton might have to have a staff sponsor and would be required to follow all school rules regarding student organizations.One of these rules is most likely going to require allowing all interested students into the club, regardless of if they are Catholic or want to be.
Another idea would be to see about using a room after school as a private organization. You may or may not be charged a fee for this and you would probably be limited to certain days and times. You also might not be able to advertise in the school other than by word of mouth.