It would be stealing.
It probably works like this:
The school reimburses Microsoft for every student copy that they distribute. So Microsoft is not losing money. The school probably gets this money by including it in the tuition. So if every student obtains one copy at most, the school doesn’t lose money either.
But if your sister has already obtained one copy, and she obtains a second copy for you, that is stealing (from the school) because now the school must pay Microsoft for another copy that your sister never paid for.
Now, if your sister has not used her own copy and decides to transfer her copy to you, then it is not stealing in that regard. It is still stealing though (to a lesser degree), because doing so would break the license agreement. Microsoft sells the student license at a discount and the student license has certain restrictions such as “this software can only be licensed to students” and “you cannot transfer the software to anyone else”. The license without those restrictions costs more money. So if you use the student copy as if it were a regular copy then you are essentially defrauding Microsoft.