Not sure if it is available in the student edition or not, but would of thought it would be. Go to tools, customize, commands and find the flex icon and add it to your toolbar.
It should be in there, but depends what Student Version you have? The Flex Feature was introduced with SW2004 (I believe) so if your version of SolidWorks is 2005 or greater, you should have it. It's possible that it is not shown in your menu bar under Insert>Features>Flex because in the last few versions of SW we've allowed you to customize even your menu bar (turn off/on items.) Go to the bottom of the menu list pull down and look for "Customize".
I recently did a short video on how to use Flex and you can view it at Charles Culp's SWTuts site: http://www.swtuts.com/?s=flex
thnx mark u helped me a lot, i have found the flex feature like u said.