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.
ell.stackexchange.com/users/42184/Andrew
I almost agree with Andrew's answer (edit: Andrew has now edited his answer to include this point).
Certainly "she has graduated in 1990" is not grammatical, and "she graduated in 1990" is what I would expect a native speaker to say.
That should be an option, and then it would be correct.
However, I think "she has graduated since 1990" is grammatical, but wrong in this case because it means something different to what is (presumably) intended.
It suggests that you don't know exactly when she graduated, but it was sometime between 1990 and now.
Out of context, it is not very idiomatic (why not say "She graduated after 1990"?), but you might see the same construction more naturally in sentences like "She has graduated since 1990, when I last saw her."
ell.stackexchange.com/users/37000/Especially Lime
It is quite possible that different educational establishments refer to graduation in slightly different ways.
In those establishments (in England) that I know about the usual form would be :"she graduated in...".
Using the word "since", as in "I graduated since five years" is a common mistake made by non-native speakers of English when the correct form is "I graduated five years ago."
ell.stackexchange.com/users/66909/JeremyC
"She has graduated in 1990" to me [native English speaker, general American dialect] would be correct only if the sentence was uttered in 1990.
If the year of her graduation was 1990 and you are speaking after 1990
then you have to say "She graduated in 1990".
If the year of her graduation was later than 1990 then you could say "She has graduated since 1990".
In this case it implies there is something especially relevant about the year 1990.
For example, imagine you are talking about some rule about how student loans are handled during bankruptcy, which applies differently to students who graduated in 1990 or earlier than it does to students who graduated after 1990.
Then it would be correct to say, "She has graduated since 1990.
"
Especially Lime gave another great example with "She has graduated since 1990, when I last saw her."
If there is not some event that happened in 1990 which is providing context for the statement, but you are just saying that the year of her graduation was later than 1990 you would say "She graduated after 1990."
ell.stackexchange.com/users/84362/Michele
If there is evidence ca.
1990 that she had not (at that time) graduated
, you could say, "She has graduated since 1990", meaning after that date.
It's not the phrasing I would use, but I believe it would be acceptable and - in that context - generally understood.
ell.stackexchange.com/users/84353/Joe Baker
Since implies an ongoing state or process which started at the point in time stated.
So you might say '
she has been a graduate since 1990'.
(this is actually a bit clunky but is the closes expression to your example).
Has or have plus
the past participle indicates a habitual, repetitive or ongoing action or state.
While graduated is to single discrete action.
Even if she has graduated several times in that period you would hardly call graduation habitual without more specific information.
So you have several options :
She graduated in 1990 :
a specific action completed in the past.
She has been a graduate since 1990 : an ongoing state which began in 1990.
She has graduated since 1990 : she graduated at some unspecified time after 1990.
This is a bit contextual and just as a statement of fact it would be more usual to say she graduated after 1990 .
In this context since is more likely to be used in the form of a question 'has she graduated since 1990?'.
even then there are only a few contexts where it would be an appropriate usage.
You tend to see it on things like official forms which are asking for very specific information.
Normally the present perfect tense (she has graduated) is not used when the time is specified.
For example
Q
: Is she still a student ?
A :
No, she has graduated.
or Q
: Is she still a student ?
A:
No, she graduated in 1990.
ell.stackexchange.com/users/32210/Chris Johns
She has graduated  [in / since]  1990.   This sentence is in my homework and I don't know which answer I should choose.  Choosing  since  doesn't make the sentence meaningful I think the correct answer is  in  even that my teacher says  since
"In" vs. "Since" in "She has graduated in/since 1990"
ell.stackexchange.com
<word-choice><prepositions>
ell.stackexchange.com/questions/183978
ell.stackexchange.com/users/84300/Ali
Graduating  is a single event, that does not normally continue over time. Therefore, you would not use "since" in this context,  unless "She has graduated since 1990", means that she graduated after that date.  You may also say "she has been a graduate since 1990": an ongoing state which began in 1990.    If you mean that she graduated in that year, neither form would be correct and "She graduated in 1990" would be the proper form. "She has graduated since 1990" could be used to mean that she graduated at some time after that date.  
If you mean that she graduated in that year, neither form would be correct and "She graduated in 1990" would be the proper form. "She has graduated since 1990" could be used to mean that she graduated at some time after that date.  