Maybe I can help here since I know a little bit about the French legal system, and I have been reading the local press reports, which have been written in a less sensational way than those in the Mail (quelle surprise). For example https://france3-regions.francetvinfo.fr/nouvelle-aquitaine/haute-vienne/limoges/noyade-d-une-fillette-britannique-a-liginiac-en-juillet-2015-le-proces-en-appel-de-cinq-personnes-2977553.html
In the French system, formally, the search is for the truth, as opposed to two opposing sides presenting the best possible view of the case. Of course, there are lawyers on both sides, but they are notionally committed to finding out the truth of what happened.
The first trial basically ended with shrugged shoulders. The court was unable to determine whether — and if so, to what extent — the death was the fault of the teachers, the lifeguard, or the people in charge of the equipment, so no blame was assigned.
The dead girl's parents appealed, demanding a new trial (to have another go at finding the truth) with different judges, and that's what's happening now. Again, the teachers are not the only ones being judged here; so are the lifeguard and the local town hall (collectively), who were responsible for the pontoon.
From what I've read I think it's unlikely that anything concrete will be established, and even less likely that the teachers will be held responsible (and even more unlikely that the teachers would be the only people held responsible). Even if they are, however, they will serve no jail time. The prosecution has suggested (from the start) that the appropriate punishment would be a suspended sentence. Still something you don't want on your record, but it would serve no great purpose to actually put them (or the lifeguard) in prison.
I think this is a tragedy on a couple of levels. Not just the death of the girl, but also the perhaps understandable inability of the parents to let it go, and the subsequent drama affecting the lives of multiple people. Nevertheless, it doesn't seem to me that there is anything unreasonable about what is happening, given that this is how things work in France. The parents are entitled to ask for this, and so it's happening. We can criticise them for that, but most of us haven't lost a child in tragic circumstances. Maybe they just want closure, and would be happy if the court decided that it was all the fault of the lifeguard, or the town hall when it had the pontoon built X years ago -- I haven't seen any evidence for the OP's comment that the parents specifically want the teachers to be found responsible.