1. How poor the school is? <-- Incorrect
2. How poor is the school? <--Correct
3. You can imagine how delightful I felt? <--Incorrect
4. You can imagine how delightful was I? <--Incorrect
For numbers 3 and 4:
If you are asking a question, the phrase would be:
Can you imagine how delighted I was?
If you are stating a fact, the phrase would be:
You can imagine how delighted I was!
Remember that you are delighted. Delighted is an emotion. Delightful is an adjective that describes things or people. If you find something delightful, it makes you feel delighted.