I have talked to two of the specialist in the Apple Service Center and they didn't even know this was an issue.
I wanted him to do exactly same environment that I am having in my iMac lab. This guy was keep saying that it is me who is having a network problem. So he was telling me to install Yosemite again. And it did not solve my problem of sharing screen.
The other guy that I talked to was patient enough to do as I have explained. If the Admin was upgraded to Yosemite and clients are still in Mavericks then screen sharing is working just fine. But if you upgrade client computers to Yosemite, then screen sharing is not working as it should be.
It has been already a month now that I told him what I am experiencing, and it is not still solved yet. He told me he was gonna report it to the relevant department. I really need to get things ready before March, otherwise my class will have to go back to the stone age again....... watching on projector screen.....