If the school is part of a group you can’t really. Usually the schools accounts on Companies house will be less comprehensive as the roll up to a group. And a lot of the big groups will be healthy re finances, it’s how this school fits into their strategy and whether they are going to make money long term keeping it.
There are a few things you can do though:
look at the info about the enrolled numbers and see if it’s going up or down. You may need to dig through previous inspection reports etc but you can usually get a feel for it. You can also ask the school directly.
Look at the groups accounts and actually read the narrative. In a lot of cases a lot can be cleaned by that in respect of direction they are looking to go - ie growing certain schools or areas, they may talk about rationality properties. Are they in acquisition mode? Have they already started to close provisions?
Look at where the school sits in the indie league tables. If the school is top 100 or top 150 and the group is making money then it’s unlikely this is a school they will close.
If you are really nosey you can look to see who owns the land, and also then whether there is a push for development in that area. If they own the land outright, it’s not a big or top performing school and the local council need to build more houses there is a chance the group will cash in.
None of this is perfect, and none will give you a straight answer but all this can help paint a picture on the risk that it may close.