"in first grade" and "in third grade" mean the same as "in the first grade" and "in the third grade" and are probably more common usage; otherwise, the sentences are fine as is. I'm not quite sure what you're asking about with the three choices "I, we, you", but if you mean substituting "I" for either "We" or "You" in the given sentences, neither would be good usage.

Yes

Your original sentence sounds totally fine to me.