Since you do not have her private key, you will never be able to decrypt any files encrypted to her key, regardless of whether they are past or currently encrypted files.
You were correct to generate a new key for your current use. You should have sent your new public key to the test score group for them to encrypt to. If they actually encrypted to your new public key, you will not be receiving that error message when you are attempting to decrypt it from the machine that generated the key.
To make sure there is not a problem on your end, you should encrypt a file to your own public key and then make sure that you can decrypt it.