Serious hackers are not, we suspect, drawn to our network or our students' work, and so we just haven't seen problems in our classes. Moreover, because our classrooms are arranged so that teachers can read over students' shoulders, the classroom setting isn't conducive to hacking.
Finally, when we have computer experts in class, we try to use them as experts--as tutors for students who don't know as much about computers and even doing class demonstrations of Internet search strategies--and that creates a positive atmosphere in our classrooms.