You'll have to ask your professor / college and check if they can provide you one. You may also attend SOLIDWORKS User Group Network meetings in your area and you may get one there. Last option is to purchase them.
I agree with Deepak These would be your best options as a student.
I got one at a user group meeting one time but it was time limited and I was not able to prepare in time due to my work.
If you are not currently in a user group it would be a good idea to join, you can get good information there - as well as the occasional voucher.
Some colleges do offer them.
The purchase price is $99 the last time I looked and that should not have a time limit.