Learn English – “In” vs. “Since” in “She has graduated in/since 1990”

Neither you nor your teacher are correct, as neither "in" nor "since" would be grammatical (at least not without a lot of background information).

The problem is with the present perfect has graduated, which is an awkward conjugation of the verb. There are few cases where it makes sense, as it indicates an action recently completed. For example:

He has recently graduated from college

He has graduated from college, but he's considering going back for a graduate degree

Some people might use it to talk about the school where you graduated from:

He has graduated from Harvard.

However, in this case I would simply use the simple past "graduated" -- which is also what I would use when talking about the time frame:

She graduated from Harvard in 1990.

Graduating is a single event, that does not normally continue over time, so you would never use "since" -- except perhaps ironically:

Our lazy son has been "graduating" since 2015. Maybe this year it'll actually become official.

(Edit) As Daniel Roseman points out

I last saw her in 1990, when she had not graduated; she has graduated since then.

is grammatical, although in a very specific context. You could just as easily say something like:

She had not yet graduated in 1990, which was the last time I saw her, but I heard she eventually did graduate.

It depends on what information you want to emphasize in the sentence.