I second (or third or fourth) that the first thing you need to to do is know what the school policy is so talk to the teacher. Find out what the deal is on all the sweets and get some background.
The second thing to do is become actively involved in the school- regularly attend PTO meetings and other school functions. Volunteer in the classroom. If you can't be actively involved with the school due to scheduling, etc than find a like minded friend who can. You can work thru and with them to effect change. The goal is to be a respected part of the community. Doing so makes implementing a change in policy that much easier.
In that vein get to know the other parents, even those in different classes than your child. Talk them at play dates, school functions, at the playground. Having other people in your court is more powerful. If you are the only once who cares about food issue it will be much harder to ask for change.
Lastly make sure you know what your end goal is-is it to ban all sugary snack/sweets? Is it to stop the celebrating of b-days/holidays in the classroom? Is it remove allergens? What *is* acceptable you? What are you willing to compromise on?
I noticed a huge increase in processed foods in our son's (small private) school when they were forced to implement a "peanut/nut free" policy on the entire campus. Now there are no more homemade flax seed oatmeal cookies, whole wheat banana muffins/cake, fruit salad, etc. No more homemade anything. The policy was revised and includes a list of "approved" snacks-all of which are store bought, processed crap chosen because they were peanut free. While I have found a few whole/natural brand alternatives it gets expensive for 12 kids plus it has to be submitted for approval!
The majority of the parents have been very vocal in their unhappiness but there was too much liability and the insurance company forced them when a child with a severe peanut allergy enrolled. At this point we are still working within the system to get the policy changed and all but an small % of the parents and lawyers agree. Until then most parents are trying to choose the least offenders from the list but some have given up so we have seen a increase in artificially dyed squeeze yogurts, "fruit" snacks, etc.
However on a positive note the school did really up their already fabulous cafeteria which has been a boon.