User3204,
Text messaging is definately the best method of getting a message to parents quickly and directly in times of emergency/school closure as over 85% of adults in the UK own mobile phone and over 90% open/read an SMS when it's received.
Local radio stations and the schools website should also be a high priority and VLE if the parents are using it. Also as mentioned a recorded info line although this could get overwhelmed if all parents attempt to call all at once. A variation on this could be a dedicated SMS text in number that generates an SMS autoresponse (this is something we offer).
One of the other suppliers of text messaging solutions mentioned earlier has a facility to send recorded voice messages to landlines and SMS to landlines in addition to the mobile, which is a great feature and caters for the few parents that don't have a mobile phone, might be worth you taking a look at that. This is something we are looking to add in future versions of our service.
One other method I've heard of but I don't know how popular or effective it is, is snowday if you just google snowday you should find their site. This is another SMS text alert system that will send out the alert for you but the onus is on the parent to keep their details upto date with this site, if they change their mobile number chances are they will forget to tell this site.
I wouldn't rule out email totally as more are more people are able to access it on their phones so could get your message early in the morning but this shouldn't be your primary communication channel.
Finally you could also try the Lollipop man, true story, local school to us used roped in the lollipop man to deal with parent enquires and answer the phones when it snowed hard earlier this year. Not sure that's in his job description but good sport and worked well nonetheless.