Just a thought... have you considered
VEX IQ? VEX IQ officially targets kids ages 8-13 I think, however my high school kids love the IQ kits when they get to use them.
EDR and IQ could probably be used in tandem to provide options to students of varying interest and ability, allowing you to address less experienced builders without having to invest in an entirely new set of equipment. I've been told (I'll double check tomorrow when I have both kits in the same place) that the holes are on the same pitch so the kits could theoretically even be used on the same robot (I'm not sure I'd recommend that, just saying I think it's possible).
EDR and IQ feature the same build style, utilizing beams, plates, pins/screws, and standoffs to assemble mechanisms, so most designs could be built equally well with either system. With some prior planning lesson plans could probably be adapted so that students could utilize either system.
IQ is nice for new designers and builders as it's relatively forgiving if a rebuild is necessary. The snap together nature means that less time is spent on assembly and more time can be spent on design/testing/iteration. It's also very beginner friendly with its out-of-the-box teleoperated control.
Also, if you do any software both kits can be programmed in RobotC, so that might be handy.