"Is it a good idea to put a hash of student diploma digitally signed by several professors on Bitcoin blockchain? Yes."
How exactly would one do that? Is the idea to send a tiny transaction to a made-up wallet, with your hash embedded in the wallet id? Seems like it would need to be pretty small, and would be somewhat of an abuse of the system. Or if that's not it, what exactly is meant by "put a hash of __ on the bitcoin blockchain"?
The hash is already signed, then so as long as you can trust the keys used to sign it (a problem not solved by blockchain), then you can get it from anywhere, any untrusted source, because the trustability is inherent in the object itself.
The OP_RETURN code was introduced by the Bitcoin core developers to address (but not necessarily endorse) the increasing desire of people to store non-financial data. The code signifies that an output is provably unspendable, allowing transactions to be pruned from the UTXO database.