Hi pianodub
I've also just started an orchestra in the primary school I teach at, and it's a lot of fun! The violins I have range from pre grade 1 to grade 2. I'm using music from the Absolute Beginners series (published by Phoenix Music) which I would recommend, as the arrangements are very flexible and geared towards this age group and ability.
I would re-iterate what others have said - keep to 1st position and avoid too many sharps and flats. Having said that, the 1st piece I did was in C major and I had to teach some of the violins where F natural was
If the school is anything like mine then I'd expect an odd assortment of instruments - I have a lot of guitars and recorders, a couple of trumpets, a clarinet and cello as well as the violins. It's a case of working with what you've got
Do PM me if you want to share ideas, as I'm very new to it all!