When I get in six flags of Texas ,can I go in and out of six flags ?

Definitely withal a season pass/membership. Im mot sure if you can with a regular ticket.

Yes they stamp ur arm

You can if you have a membership or season pass. Not sure if you can with a one day ticket, used to be able to.

Once you go into the park, you just need to get your "hand" stamped. That will allow you access back into the actual park, but keep in mind if you go out to the parking lot for any reason, you are required to go back through metal detectors. The stamp doesn't allow you to skip the metal detectors at any point in time.