Hi Chykie,
got another idea for you, then I'll leave you in peace!!
Could you get a branch or some twigs and tie them together to make a tree and tie dangling leaves on to it, each with the word 'welcome' in different languages. It could symbolise the tree of knowledge, growing together, separate leaves but from the same tree (all different but all the same) etc.. Does't have to be a tree, could be petals on a flower, leaves on a plant. You could make the background like your school playground or put a simple silhouette of your school in black at the back. The leaves could be different shades of green and would stand out nicely then?! You could attach the 'tree' by tying rubber bands around the trunk then pinning the rubber band to the board.
All out of ideas now - I don't have that many!!
SN