I entirely sympathize with your dilemma. I've been trying to teach Software Engineering for games in a University context for nearly 8 years in what I would consider quite a progressive institution (post 92). I got my way in the end (sort of), thanks to a lot of buy-in and help from staff, but there were threats of physical violence, committees disbanded and no end of political shenanigans (my back probably has more holes in it from the daggers than a tea-bag). The Uni now has a policy where they *expect* trouble and problems, and aim to fix it, rather than prevent it from happening in the first place (hooray!). Mind you, We did only manage to get one room (24 pc's and PS3 dev kits). It's doubly complex because of the JANET agreement, but I won't dwell.
So, with that experience, and relatively recent experience of working with schools, I would favour the cable issue over the PC security issue. The Pi can be connected up safely and the equipment needed for each case could even be rented or supplied free (I'm hoping to do that). Yes, you will find jobsworth's who object for no end of reasons but the alternative - allowing unfettered access to the PC's in schools would, in my opinion, be next to impossible and probably why the Pi is here.
Yes, schools are worried about the cables and the like (quite rightly), but in my informal survey, they seem to be willing to get over that for the freedom.
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