Does anyone on here use a sling for the school run? Would you still use one in bad weather - wind rain etc. I'm not a big sling user/wearer so not sure if it's unfair on baby to be in a sling as how do they stay warm and dry?

You can put a large coat over a front-sling so baby is snug inside.
Some ponchos will go over baby's head and you own head.
Baby in a hat and all-in-one snow suit in a sling can work.
I have not had to have a baby in a sling in bad weather as I have a buggy seaty available, but can be done and many parents don;t have a buggy at all and just do 'babywearing'.
There are babywearing forums and facebook groups who might be able to help you further.