You can block the area not being printed with masking tape. It just comes off
when you're finished. I remember doing this in college. Use extra wide tape.
You can buy it at a paint store.
I used to do screen printing commercially and we used the blue photo
sensitive film and something like rubber cemet to block the rest of the
screen. Cleaned off with a paint thinner. I also have a lesson where you draw
right on the screen with white glue. I tried it and it does work, but can't
make small details. It should wash out with soaking. I haven't tried to wash
my sample out.
I bought large embroidary frames with the intent of introducing some form of
screen printing with 5th grade, but havent' found the time yet. I love
printmaking. It's so much fun for the kids. It's such a surprise when they
see their print appear!
Good luck, JH