What happens when the snow appears in Barcelona?

Barcelona has a Mediterranean climate with soft mild winters, so the snow is something very rare to see here. Yesterday the first snowflakes appeared and this went on set off alarms.
All of us can remenber the big snow on of (<em>or</em> from) March 2010. That was unexpected. I was working in the school when the first snowflakes appeared. It was amazing for the children who had never seen the snow. At midday, while we were having lunch, the snow storm intensified. We could see through the window a white surface layer on the ground. At three o'clock, before opening the school again, we took a decision: to phone Educational Department to ask permission to close the school, but they didn't allow us to close.
The following hours were a chaos (<em>or</em> chaotic): car accidents, we couldn't walk in the streets, my sons' school closed, parents couldn't arrive to pick up their sons (<em>or</em> children, unless it's a school only for boys), roads closed..
Barcelona is not prepared for the snow, so went when the first snowflakes appears appeared, alarms went on off!

English can be a strange language. When an alarm goes on, we say that it goes off! When an alarm finishes, you would say the alarm stopped.

For "remember the big snow" it would be better to say "...the big snowfall" or "...the big snowstorm"

You can use "snowstorm" as one word, although "snow storm" is OK (same for rainstorm, thunderstorm, etc).

If you have one son, you would say "my son's school". If you have more than one son at the same school it's correct the way you wrote it (my sons' school)

Very well written. Stay warm!