Hi Jitendra,
Mary responded to your second requirement.
I will take #3 which is very similar to a recent forum discussion HERE. You need to set the permission of the student's role to prevent "Reply to posts" of that particular forum activity. Students can still start a new discussion but cannot respond to other people's posts.
When the "certain date" has passed, you need to change the permission for the student's role of that particular forum activity again. This time you will set "Reply to posts" to allow and "Start new discussions" to prevent.
With kind regards,
Hartmut