I'm new to the list but I will add a little something here that is in place
in my kids school.
 
When there are parties, they ask that non-nut products only be brought for
snacks.  If there is a child in the classroom that is allergic to nut
products they ask, by sending a note home at the beginning of the year, that
the children staying for lunch avoid peanut butter, etc.  Last year my
daughter (age 10) had a child in her class that was highly allergic, even
smelling it could cause a problem, therefor we sent no peanut butter and jam
sandwiches (her staple).  If the children forgot and brought nut-products,
that the teachers were aware of, then they were asked to eat in the hallway
as the classroom would be contaminated (for lack of a better term).  Once at
the beginning of the year dad made the lunches and or course put in peanut
butter, and our daughter brought it home exclaiming that, She couldn't eat
that in the class!.  We at first thought it was the teachers decision but
after questioning our daughter she said she wanted to stay with her friends
for lunch and not eat in the hallway. This year she can take anything she
wants except for on Thursdays.  That is her lunch hour for her Safe Kids
course and the same child is taking the course, so Thursdays no pb
sandwiches.  They also keep a close eye on the younger children in school as
they have a NO SHARING lunches policy.  The older children are harder to
control cause they are not monitored as closely during the lunch hour.  We
have yet to have a problem with these policies.
 
As some parents do not read labels, since they have no need to, there can be
some mistakes made and stuff sent in that is not appropriate BUT the
children can be taught to avoid sharing, etc.  I have a 4 yr old that has to
avoid gluten (wheat, rye, barley, oats and by-products of all these) and he
is already aware of what is not on his list and what is.  When it is
questionable at school he will not accept it.  Although his intolerance is
not so severe as anaphylaxis I do send all his food with him to preschool
and daycare.  Some planning and working together with the teachers and
school has allowed him to participate in all activities.