I have downloaded Xilinx 14.7 recently to implement partial reconfiguration on spartan 3E fpga. we need to indicate the project as partially reconfigurable at the beginning itself. but in 14.7 version of planAhead, that option is not available practically. are there any other tools to implement partial reconfiguration? is it not possible in student version?

Partial reconfiguration is quite an advanced topic, so I wouldn't be surprised if the student version didn't support it.  Certainly in the past is was only available in the paid-for version of the tools.

Also, S3E is probably not an ideal choice for experimenting with partial reconfig as it glitches while reconfiguring, and you can't use ICAP:

https://www.xilinx.com/support/answers/18416.html