Looking for cipher that uses one ciphertext I'm new to Cryptography so please free to edit my question for clarity and add the appropriate tag. 

I'm trying to research if the following is possible. Suppose 


John has a message that he encrypts to share with more than one
recipients.
The message is different, i.e. it's unique to the recipients
(though the recipients don't know that).
The recipients must each be given a unique password so as not be
able to decrypt each others message.


Questions (sorry for more than one question, but they're related):


What approach should I take? Links to references or further reading
will be appreciated
Is the one encryption with different decryption the wrong approach to
achieve goal 3? What approaches would you suggest?


The objective are two fold:


allow a teacher to easily (hence 1 ciphertext) encrypt "answers to exam questions" (text and
binary data e.g. images) to distribute to selected students. Reason
for not allowing anyone outside of the selected students is to
prevent students who haven't taken the exam to know the answer thus
allowing reuse of the exam questions.
allow students to see teacher's comments and feedback specific to that students' attempted answers. Since students answered differently, the students need not share the same password and are discouraged from using the same password. This also helps teacher track which students leaked answers to future students.


Thanks

Update: I've bought Bruce Schneier's book to learn up on terminology and concepts so I won't be at a loss with terms like ciphertext.
There are several systems that sound vaguely similar to your description.
Would either one (or both) of the following systems work for you?
If not, what exactly about systems is unsuitable for your application?

deniable encryption

You give a unique key to each of a bunch of people.

You think up a unique plaintext message intended for one and only one person.

Later, you publicly publish some huge encrypted file (one ciphertext).

Each person uses their own unique key to extract their own unique plaintext message from that file.

It's not possible for any one person (as long as everyone keeps their own key secret), or even several of the intended recipients working together, to find out if there are any other messages in the huge encrypted file, much less exactly how many messages there are or what exactly the other plaintext messages say.

deniable encryption

File intended for several recipients

Each person has a unique key.
(Typically each person has their own private key, generates a public key from it, and each person publishes the public key or gives you their public key some other way).

You think up a plaintext message you want to send to some, but not all, of those people.

You make up a completely new one-time symmetric key that no one else could possibly know.
You encrypt the plaintext message with this new key, making one large file.
You encrypt and re-encrypt the one-time key multiple times, each time with the key (typically the public key) of one of the intended recipients.
You end up with lots of short files, one for each intended recipient,
and one large file that is the same for everyone.
The standard OpenPGP system archives that large file and all the corresponding short files together into one slightly larger file.

Then you publish that slightly larger file (one ciphertext).

Each person uses their own unique key to extract the one-time symmetric key, and then uses that key to decode the large file.

Then every one of the intended recipients knows there is one and only one message in the large encrypted file, and they all know they are all reading exactly the same message as all the other intended recipients.

It's pretty obvious how many intended recipients there are for a file in OpenPGP format, even to people who are not a intended recipient.
But there are work-arounds to camouflage exactly how many (other) recipients there are, even to someone that is a intended recipient and can decode the file into readable plaintext.

" Hash decrypts key, key decrypts cipher... why? "

" https://security.stackexchange.com/questions/8245/gpg-file-size-with-multiple-recipients "

Information leakage from the ecryptfs filesystem

" How can two different passphrases unlock the same content? "