of course, you would need to do more in chrome (and windows/osx/ubuntu generally) to stop traffic to a site if a student knows what they're doing. that's not the point. the point is: we have this control in place. we've agreed it's working well enough. people can bypass the control simply by using firefox. to avoid adding overhead, we ditch firefox (for now). it's that simple.
as for future-proofing, that's a luxury. ...and part of why it's a luxury is that some goals ("make all traffic to any porn sites impossible on our school network") just aren't going to be met by budget IT.
re: BYOD, for that i go over to the armchair tech purist side i'm afraid, and just say "well, you allow that, so you need to get over that they can use VPNs and stuff. you're not DOJ or some wealthy corporation with important IP assets and equally 'important' VIP execs that insist on bringing their OSX 10.6 MBP to work. you don't get to have all the cool controls that might allow BYOD. sorry."