Electronic speed control circuits are now available for students to use as the basis for their own speed controller.   The circuits have been developed by Paul Gardiner at Finham Park School, Coventry, UK.   Paul’s web site has detailed descriptions, schematics, component lists, Circuit Wizard files.  You can also purchase ready-made PCBs

http://www.paulgardiner.dsl.pipex.com/EiSS/EiSS.html?ScalextricPCBs.html