But then, I think, who could possibly come close to matching the dedication and personal commitment that you have brought to this school, your Alma Mater. You will probably be the last member of the Community to serve as Principal, the last IHM presence since 1906 in an administrative position. It does make me pause to consider what your leaving really means to us in the Community
Yes, it's grammatically correct to use it because
leaving is being used as a noun - more specifically, a gerund, which is basically a verb ending in -ing used as a noun.
It's called a
possessive gerund. In this case, you can also use
what **you** leaving really means to us but in some cases, the lack of the possessive gerund can change the meaning of the sentence.
Imagine that the sentence is about a woman and instead it reads:
It does make me pause to consider what her leaving really means to us in the community.
Another more common example of this is when
doing is used as a noun, e.g.
The mess in the kitchen was his doing.
An example of the two forms carrying different meanings:
I like the girl's singing. I like the girl singing.
In the former, you like the act of her singing. In the latter, you like the girl who is singing.
For a more detailed explanation on the two forms you could use in this sentence, see this answer.