I'd say stick with QT, as you can stream either Real or QT (and wmv if MS ever decides to support proper mpeg4) and the command line version of the server is free, x86 compliant, and OSS (sort of
In any event, it's pretty easy to hack filter.php to support another video type (or any sort of embedded file).
As far as the comparison, the quality issues between the big streamers (QT/Real/WMV)are really moot, imo. Having beein involved in streaming since it became possible (what was that, QT4? Real 5?) the video/audio quality at bandwidth x jumps of one vendor are always transitory.
The main advantage of Flash is the smaller size of the player, the quality at a given bandwidth is presently lower than Qt and Real's mpeg4 (and due to the player size issue probably always will be), but it is easier for folks on modem connections to download the flash player if they don't have it, by a factor of about 6 (1mb vs ~6mb).
However, if most (or even many) of your students are on modems, you probably don't want to use any video at all, in which case a pure audio stream (and distribute the QT installer on CD to the class- you can get a free license from Apple for this) is going to give you more quality at a better bandwidth, as Qt's streaming audio codecs are vastly superior to Flash's hoary mp3.
IMO, flash makes the most sense as an http streaming/progressive video player (no special server required), due to it's ubiquity and size of the player, but when rtsp is called for, esp. in an educational setting-where you have a known client/student list and method of communicating with them other than via the streaming video page, ie they are not tranistory viewers who will leave if it doesn't work right away, QT is a better choice.