We were a group of two teachers and 11 students and we booked two nearly identical flats in the same building. The hallways of the building could use a little freshening but the flats were clean, well-equipped and furnished, close to the North Greenwich station. Quite easy to get around London from there. The Coop supermarket is just around the corner, so that was huge for us. The staff of the building were kind and helpful. All in all, great value for money.
Tip: Loads of restaurants in London no longer accept cash! So if you are a school group and bringing younger teens, make sure that they have a bankcard before you travel!