Both of the following websites offer concept-specific teaching videos that are appropriate for middle school students. Do a search of the site for the concept you want to practice (e.g. 'dividing fractions'), and select the related videos you want to watch. Both sites are FREE and have a wealth of information.
FREE ~ PreAlgebra, Algebra 1 & 2, Geometry. Also includes practice tests for ACT & SAT www.mathplanet.com