the boarding school : black lagoon

I'm stuck at the part where you have to open the door in the caves, and Marcos said he forgot the right key.
He mentions the "z" key, i think, but I have no idea where it is.can anyone help me?

I don't have that either. I don't think you're meant to go there yet.

if not, then i don't know what to do. :\
if you find a way, please tell me

Erm... I went to the door where your sister says "Isn't that like mom's tattoo" (I had to look around a fair bit though. Then you need to run because an alarm goes off. After that you're back at the school.

go back to your room and get the key

its actually the big steel key thats in your inventory, approach the door and look in your items and click on that to use. it should open the door