Which way to program this?

[Mark Neidorff <mark@neidorff.com>]

I am considering writing a "visual department scheduler" for schools.  The 
concept is similar to an appointment calendar, but I'd like to include drag 'n 
drop functionality.  A supervisor is provided with teaching session objects 
that meet in fixed time periods (call them 1-12).  The supervisor drags and 
drops them onto the individual teacher's schedule.  This application should be 
cross platform, able to run in windows, mac and linux.  So, it seems easy to 
me to conceptualize this in a web browser, but  what language/development 
envoronment would be appropriate for this project?

Any thoughts are welcome.

Mark