Making a NAS out of an old laptop

So this summer I will be handling a project with a school in Haiti to get the a computer lab. We worked with my dads company to get older laptops that they would normally donate to local schools to be given to us for this project. All of the laptops were very nice machines (i7 based) except for one that was an old centurion. Recently we thought of the idea to get them just a basic file server so that all of the students files could be backed up onto a separate machine in case of a crash or mishandling of a machine(very likely with the conditions down there). So I thought about trying to turn the older centurion machine into a file server. What would it take to make it into a file server. The only files on it will be small documents so space shouldn’t be too much of an issue. I plan to get a larger/faster drive and am just wondering what I should do to make this a server that multiple laptops can connect to at a time to backup/store files on.

Edit: The main laptops will be dual booted with windows and linux depending on what program they will be using at the time. I would like the server to run a version of linux as we do not have the funds to buy any more windows keys.

You could try freenas that is an os that run on a flash drive. If you go to the forums expect to get chewed out because you don't have server grade hardware but I managed to make a very basic nas run with a netbook and its internal drive then you could just make one folder that isn't password protected or anything and have them all back up there files to that. Also the os is based on FreeBSD so its completely free.