While I do not like it, it seems to be allowed to work on another task while another sits in the review queue.
There are however some things you need to keep in mind and a single rule:
You may not make a pull request, HOWEVER you can push your code to your personal github branch.
If a student who claims it and works on it (in the proper order) and the task is no longer available, the student who claimed it prior to starting work gets priority, and your contribution will not be used in this case. Put simply, you do this at your own risk. Use it as a learning experience.