Formal:
* Croeso i'ch ysgol. (This is the most direct translation, meaning "Welcome to your school.")
* Croeso cynnes i'ch ysgol. (This adds "warm" to make it more welcoming, meaning "Warm welcome to your school.")
Informal:
* Croeso i'r ysgol! (This is a more casual greeting, simply meaning "Welcome to the school!")
You could also use "Hwyl fawr!" which means "Have fun!" after any of these greetings.
It's important to consider the context and who you are addressing when choosing the appropriate greeting.