I am going to get a server/software for my school (it's a small language school) and I do not really know much about it. I need to improve the administration work.
Could you please give me some advice here?
What I should look for and what's the best for me?
I would really appreciate your help here.
You are going about this the wrong way. You need to figure out first what improvement you need to implement, then find out how to do that. Just getting a server will not magically fix things. What procedures do you want to move to a central place, what is the setup now, quite a few things to sort out first. A true server is not a cheap thing.