who has to take the responsibility?

This is very complicated. They should have contacted you first. Your school might think that you haave to take responsibility though, becauseit was meant for you. Now was'nt it? So I don't think that there's no one to blame. These things happen.

It is not the school's fault if a bank allows someone to negotiate a check that was not written to them or transact on an account that is not theirs. That is the bank's fault.

The school sent you your statement just as you asked. Of course they sent it regular mail. Of course they didn't contact you before they sent it. That is standard procedure.

Financial statements are mailed all the time. Are you talking about a financial statement, or a check??? Either way, nobody but you should be able to cash it. If it is a check, then you can contact the school and see if it's been cashed yet. If not, you can tell them it is lost and perhaps they can put a "Stop Payment" on it if you think it's been lost or stolen.