I have a secured loan with a credit union, and I have been paying in time since I opened it 6 months ago. Would it be possible for them to release the rest of the secured amount? I mean I would love to use that extra cash on my school books and other school supplies.

When I applied a loan with them, my credit score was at 612 back in April and as of right now (October 11th) when I pulled up my last report with transunion at myfico.com, I had a score of 699.

Would this be possible?

You'll have to ask them if they can convert the remaining balance to an unsecured loan. They might.
Another option would be to use the security deposit to pay off the remains of the loan and take the left over cash.