Subject and Performer

I think subject and the performer of the verb of a sentence are not same always.

I eat rice. Subject:I Performer: I
Rice is eaten by me. Subject: Rice Performer: me.

Am I correct?

I am supposed to be a student.
Is this sentence correct? If correct then could anyone please explain the sense of the sentence?

Right, in a passive sentence, the grammatical subject is not the performer of the action.

“I am supposed to be a student” is correct. More context is needed to be sure of the intended meaning. The person may be asserting the fact to someone who is not treating them like a student, or they may be reminding themselves that they ought to behave like a student, for example.