You may have heard some discussions regarding the safety of WIFI use in schools as a result of the May 21st Panorama on BBC WiFi - A Warning Signal. The program compares WiFi, mobile phones, their cell phone tower "masts" and other forms of radiation. The program proposes that further studies be undertaken. The inference might be made that we should stop using WiFi until its safety is proven. Here are several web sites representing a variety of views on the topic.
Although it seems to be breaking news, this has been an item for concern in the UK, USA and Australia for several years.
In regards to the BBC program, this retraction was issued: "The statements attributed to Sir William Stewart, Chairman of the Health Protection Agency (HPA), in The Independent on Sunday are not his. Sir William is being pressed by lobbyists to condemn WiFi and is unprepared to do so. He has not taken a position on WiFi. WiFi devices are of very low power, much lower than mobile phones. The HPA and Sir William have always pressed for more research into these new technologies. The only firm precautionary advice issued by the HPA is about children's use of mobile phones." http://www.hpa.org.uk/radiation/default.htm
Many schools, on issues of children's health in the US, defer to the guidance of the American Academy of Pediatrics (AAP). By building policies on their recommendations (e.g. your child should spend x days out of school after being diagnosed with y) you are working from rigorously reviewed, evidenced based, broadly accepted guidance. You can suggest to parents that the school follows AAPs guidance and that you would be happy to consider recommendations from them.
Also, the WHO report seems an authoritative source: http://www.who.int/mediacentre/factsheets/fs304/en/index.html
These kinds of things come up time after time. Whether it is emergency preparedness, peanut allergies, or pool safety someone is going to throw a recommendation or research at you. If your policies are built on top of recognized professional resources, and you stay consistent, it can help mitigate the amount of tire spinning these often sensationalized media reports can cause. Referencing outside, professional resources can also give the parent or group another target to go after, instead of the school, especially in the case where a parent is a subject matter expert and you have little chance of matching their knowledge or persuading them (and especially in your case where they are bringing an internationally recognized activist).
I have found the WHO fact sheet helpful in explaining the issue to parents and would recommend sending that in response to other parents who may be concerned. The CDC maintains a fairly comprehensive listing of current EMF/RF research but most of it is very technical. http://www.cdc.gov/niosh/topics/emf/
Even though it comes from a non-neutral party, the Wi-Fi alliance has a good brochure: http://www.wi-fi.org/files/kc_13_WLAN%20and%20Health_7-20-05.pdf
One of the advocates against WiFi is http://www.safewireless.org/