Translation:She was my classmate in middle school.
I agree entirely, there is nothing that signifies past tense. Yet google translate comes up with the same answer if you cut and paste the answer (English to Chinese).
And this is what Wiki Chinese grammar says:
"While not strictly tied to any "tense," the 是⋯⋯的 construction is frequently used when asking or telling details about the past."
Well if you put the Chinese character sentence "她是我的初中同学" into Google translate, it comes up with:
"She is my junior high school classmate." (present tense)
But if I put in the English answer "She was my junior high school classmate," (past tense) it produces the same Chinese character answer: "她是我的初中同学"
So I just don' t know. I find it impossible to be certain of the past tense doulingo interpretation as well with this sentence.
Like you I am only trying to figure out what is going on here.
I have noticed the same thing as you. DuoLingo and Google will translate the same Chinese phrase as both past tense and present tense in English.
Chinese verbs will not change state, instead we add a time adverb or an aspect particle to the sentence to indicate past, present or future
You have to remember, unlike in English, we do not have past tense for verbs in Chinese grammar. Both "is" and "was" is written as "是" in Chinese.
"是" in "她是好老师" could be "is" or "was". We really don't care which, because it is presumed readers can find out the sentence is in past or present tense by looking into the context.
The hard part about translating a single sentence on DL is that we have no context. If we heard a middle school student say this, we'd know it was present tense. If the person was an older teenager or adult, then we'd know it was past tense.