1. "She should do well on the test" is probably an expression of expectation, but it could possibly count as an expression of the speaker's feelings about "her" duty to perform well. "She should have done well" is possibly an expression of the speaker's feelings of disappointment at learning that she did not do well, or an expression of confidence that she did in fact do well.
2. "The letter should have been sent last week" is equally ambiguous.
3. "James should have been told..." sounds very much like an expression of disappointment at a failure of communication.
4. "The class should be divided in half" sounds like a directive, not a conjecture about a situation.
If any of these sentences occurs in a written text, the meaning will be clear. If they occur in speech, the manner of speaking usually provides sufficient clues. This is why the study of isolated sentences, though not without merit, provides only part of the picture of the communicative force of an utterance.