Karibu shuleni kwetu.
Here's a breakdown:
* Karibu: Welcome
* shuleni: at the school
* kwetu: our (literally: to us)
You could also use "Karibu shule yetu", which means "Welcome to our school". The difference is subtle, but "shuleni kwetu" emphasizes the location ("at our school"), while "shule yetu" simply means "our school".
Both phrases are perfectly correct and understood, so choose whichever you feel is more natural.