All are correct, but have different uses. "She speaks Farsi." - This isn't suitable for the context. This just means that she has the ability to speak Farsi." "She spoke Farsi." - You could use this. "She has spoken Farsi." - This sounds strange, as if she used to be able to speak Farsi but can't anymore. "She has been speaking Farsi." - This is also possible in the context. It also implies the was speaking Farsi for an extended period of time before the teacher came in. "She is speaking Farsi." - This means that she is speaking right now. "She was speaking Farsi." - I'd reckon this is the most natural option in the context. "She had spoken Farsi." and "She had been speaking Farsi."- You might use this when telling a story (like you telling us what happened on italki), but they wouldn't have been appropriate at the time. So: "She has been speaking Farsi" and "She was speaking Farsi" are the two most natural options in my opinion, but "She spoke Farsi" is also possible. "She was speaking Farsi" is what I'd use. I suspect British English speakers would be more inclined to use "She has been..." while American English speakers would be more inclined to use "She was...". I find that Australians are generally somewhere in the middle of American and British English speakers when it comes to the use of past or perfect tenses.