First, the bread itself. I'd suggest a smooth, forgiving dough like
challah -- and it's close enough to "white bread" that the teenaged party
guests shouldn't balk.
Quantity -- it sounds like roughly a loaf of challah to me. I'd probably
go ahead and make a recipe (2 or 4 loaves) and use whatever was needed for
the party loaf, then shape the remainder into a loaf or some rolls,
whatever suits the quantity remaining.
Technique -- I'd stick to a 3-strand braid; larger numbers of strands make
pretty presentations, but they would get too big for your 12" pan very quickly.
The ropes probably only need to be 1" to 1.5" in diameter. Challah dough
is very nice for braiding -- if you find one strand is too short, just
pinch more dough onto the end of it. The joint disappears in the
rising. Start with 18-24" lengths and see if that will be long enough to
make the circle you envision. Extend as needed.
The braiding itself -- if you keep the strands pulled up close to the last
criss-cross, rather than leaving everything more-or-less vertical, you'll
get a tighter braid. I do this by laying the strand I've just moved so
it's about perpendicular to the loaf; it looks weird in progress, but give
me a much prettier result and a denser loaf that holds together well when
sliced.
Pan ... don't know whether your 12" pan has sides. If so, you might want
to consider a pizza pan or cookie sheet so the outside of the loaf stays
round and the braiding gets shown off prettily.
Form in the middle ... probably only needed if the center hole is starting
off very small (which could happen if your braiding strands are pretty
thick). If needed (or just wanted!), a small cereal bowl, or even a juice
glass greased or sprayed on the outside, is an effective "spacer."
Here's a trick which produces a gorgeous chestnut glaze on challah: after
final shaping, brush with beaten egg diluted with a little milk. Let the
loaf rise UNCOVERED (drying out the glaze somewhat). Just before baking,
brush AGAIN with the egg mixture. If you want to sprinkle seeds on top, do
it now. (acknowledgement to Secrets of a Jewish Baker) Bake and enjoy a
beautiful bread!