Pronunciation and the usage of 啊
I am not sure about your teacher's opinion, it seems like she would want to use 呀 for the desired pronunciation yà.

Though, 啊 does have many tones, it can express feeling of surprise in first and third tone, it can express the meaning of asking in second tone, it can express meaning of adorable, getting it or surprise in fourth tone.

It can also mean a lot in its light tone.

You would be able to find all those in a dictionary, but keep in mind, it is not that strict, so use your intuition.