To expect my son's school to ring me back because I want to report older boys with a flick knife in the school grounds

[NL3]

I've told the school this - and I have a picture of the boys on my mobile phone. Surely they should be interested enough to get the details or am I being unreasonable?

No you are not being unreasonable at all. Ring them again and ask to speak to the head.

ring them and say if you have not heard back in 10 mins you are calling the police instead.

Sod phoning the school - phone the Police now