We stuck our then 4 and 6 year old in ESF (through Crystal) back at Easter 2013. It really was the only ski school in 1800 as far as I could tell as there were no non-ESF meeting points there that I saw. The rest were all up the mountain in Plagne Centre or elsewhere, although to be fair it was only a short lift ride or two to the centre. Never actually used the bus.
The ESF didn't appear great but it wasn't terrible either. The groups weren't huge probably because it was Easter, and the instructors weren't mean as far as I could see. One was more "lighter" and humorous than the other (fortunately this was the one teaching our four year old!) and there weren't many (any?) other English children in the group. We stalked them a few times on the mountain and there was an awful lot of "follow me" going on.
Our six year old was really challenged and did lots of off piste down gulleys and up the sides of runs, and a few red runs on what was her first week on snow, albeit having completed a full series of lessons indoors at Hemel first.
Our four year old, same set of lessons at Hemel, we discovered all over the area too, mostly on blue runs although the main route back into 1800 is a rather steep red run and it was funny to watch them all topple like dominoes (no doubt exhausted!) whilst we were waiting to pick them up! She started out in the snow garden and stayed in 1800 on the first day but the fact we were finding her all over by the end shows how much they learned.
Both girls really really enjoyed themselves (and both are quite sensitive souls) and their skiing improved far beyond what we'd been expecting so whatever it was they were doing, even though it didn't look great, it clearly worked!
We had a couple of private lessons for ourselves through Reflex, and our instructor was really good. She was happy to meet us at the lift in 1800 too. I can really recommend them.