My parents are both teachers and this sort of thing, which you would think would be gratefully received tends to be a bit of a double edged sword. Very much like the Tesco computers for schools programme, it will require far more effort from the teachers than you'd expect.
In this case what's happened is that Google have got some pretty good advertising but:
1) There are (as far as I can tell) no monitors or keyboards, or mice, possibly even no PSUs supplied.
2) There is no training for teachers supplied - this will have to be done on the teachers' own time (not entirely popular, considering how much overtime teachers already do)
3) There is no kitting out of particular rooms to handle the r-Pi class sets
4) There is no network infrastructure supplied
5) It's not clear if there is any educational material supplied and if there is, if it's suitable
6) There is no space in the curriculum to insert new classes, so it's highly likely that just a few kids who can stay after school will be able to get any use of them. Also note: Most kids who live a bus ride away from school can no-longer stay after school because they have to get the school bus, not a normal passenger bug.