I am applying to graduate school for art education.  Do you have advice on how to put my portfolio together? Should all the slides be in one concentration or of many mediums?

Congratulations.  What a wonderful career you have chosen.  My best advice I have to offer you regarding putting a portfolio together is to contact your chosen College or University Art Department and ask them.  I think you will find that they will want from 5 to 30 slides showing a representative sample of work in a wide range of media.  I think it is good to also look at the your state art standards for K-12.  There are classes available at some Colleges and Universities that teach you how to put together an effective portfolio.  You can also surf the net and look at the admission requirements for art education graduate students.  They will often include portfolio requirements.  If you have taught any art workshops as an intern or T.A. and you have student samples I would also think about including those in your portfolio.