Astham UK provides a good place to start for clear advice. The site is well set out and it is easy to navigate.
Here there is information relating to;
http://www.asthma.org.uk/how-we-help/teachers-and-healthcare-professionals/
http://www.asthma.org.uk/how-we-help/teachers-and-healthcare-professionals/schools-and-early-years/
There is specific advice relating to schools;
http://www.asthma.org.uk/about-asthma/