The answer is YES! I have used Client version 12.5 with server 15.5.
But the problem is that you will keep on getting the pop-up that says "An update is available for download" like every 2-3 secs, literally!
And you wont be able to take it beyond a certain point, and you will have to update the client because it will get irritating.
I had upgraded the clients to the latest version and they are way better than the 12.5 but the reason why I know it will work is because on the test system I have, I kept the client on v12.5 to see how it will react and trust me, it is a pain to keep the client in the older version.