My Russian teacher, claims that when pronouncing "часá" it should be pronounced as: "чi-сa" with an "i" instead of an "a". I have looked around and it appears that the pronunciation hinges around: "cha-sa". You are native Russian speakers and could perhaps tell me what is correct and what is a myth.
Your teacher is absolutely right. According to the rules of pronunciation of the Russian unstressed vowels,
щin an unstressed syllable is pronounced as the letter
и, that is as [ɪ].
Have a look:
часы́ - [t͡ɕɪˈsɨ] 'clock'
часовóй - [t͡ɕɪsɐˈvoj] 'sentry'
щавéль - [ɕːɪˈvʲelʲ] 'sorrel, dock (plant)'
счастли́вый - [ɕːɪˈslʲivɨj] 'happy'
That is why the word
часá is pronounced as [t͡ɕɪˈsa].
Do not think of a vowel reduction as simply a vowel "replace". Honestly, every time when I see a phonetic transcription, I can't believe that it's the same what I speak.
Actually, the rule of reduction is really simple: speak neither
ч-са, where the dash stands for "some noise" left between spelling
с. No speaker really cares if it's closer to one vowel or another. Just do it quick and never stress.
As a native speaker, I can assure you it definitely is pronounced as "cha-sa". And I really can't pinpoint the myth origin.
Considering that a in ча can be both stressed and unstressed, it really is pronounced differently depending on stress. But it's still is pronounced as a, not as I. It's just less opened and bright.