I have also heard that you should not show "too much" in your portfolio. Does anyone keep one portfolio for themselves and then pull pieces from that to show the school officials?
This principle applies to many things... portfolios... college essays... resumes... even documentation of problems at school. A person keeps quite a bit, then can "filter" to choose the most applicable items to share for the given situation. For a portfolio, a parent may wish to talk through the selection process with the child, as the child will eventually manage their collected works... it may be good for a child to begin learning this application of decision-making skills early as it may help to build the student's internal locus of control.