DropBox

I am trying to create a Dropbox style folder for students to place files in.
I basically need them to be able to write and read (that's optional). I
don't want them to be able to delete files that belong to other users, but
it is ok for them to be able to delete their files (I'm trying to use
creator/owner for this). When I use special permissions to remove delete
from their group they lose the ability to write.... Can someone please tell
me how I need to configure the permissions to accomplish what I'm trying to
do?

Thanks,
Aleise