I think I understand what you’re trying to say, and indeed the sentences would have different corresponding English translations.
Either you can formulate a more specific case, for a specific subject, e.g. a student:
“Lo studente si dovrebbe fare i compiti da solo”
“The student should do his own homework”
Or a more generic case, where there is no subject specified, just the impersonal “one” form is used, hence the “si” is conjugated to follow the associated object of the sentence (which can be singular/plural and masculine/feminine):
“Si dovrebbero fare i propri compiti da soli”
“One should do ones own homework”