STEM programming kits

[HTCPCP]

One of my schools are looking at replacing their old Lego Mindstorms kit, for predominantly year 5/6 ages. The school have ruled out the lego WeDo, as a bit too simplistic for the more able kids. What are you guys using? I have seen (although only in demo mode, and not in the classroom environment) Vex IQ and the K'Nex kits, although not come across any schools that are successfully using them.