Hi these pictures are great!
I'd love to add a few explanations.
I visited Wellbury house earlier this month and i was so confused when i got there to see that it was still an active Orthodox Jewish school. The fact that the old building was so ruined and judging from the pictures above i was stunned to find boys appearing to be actively going to school and what i can only presume, are infact living there. I was not able to go into any of the buildings because of this. The newer building of the two as you said appeared to be empty dorms and whilst i was there the building was full of pupils who were looking out of the window at us very perplexed by our presence. The pictures of the dorms don't look in great condition but they are definitely lived in by those pupils.
Also, the older building of the two appeared to have some power source as a pupil was on a phone connected to the wall and in the building they were cooking and an extractor fan was on. Eventually an older man came out and asked us to leave saying that it was a private place.
I was confused for a number of reasons. Firstly, how could a school be run in such a terrible state? Surely it can't have passed basic health ad safety tests. It was closed in 2002 because it was in a terrible condition as you noted. I'm guessing it was in a pretty bad way then also. Secondly, why did all the pupils seem so perplexed to see us? I was also suprised that they did not appear to speak any English at all and why were they all male?
After researching for a while i discovered that at age 13 boys who follow orthodox Judaism attend schools like the one this school used to be and currently still is. It was and is common practise for these pupils to travel from abroad in a type of boarding education. Reportedly education of the Yeshiva education institution is highly religious and barely focuses on basic Maths and English skills. However, despite all this it still did not make sense as to how this school was still running. Again, i discovered that Yeshiva education like Islamic education in the UK is highly unregulated by the department of education. This means that they are usually not registered and therefore unlicensed and essentially exist illegally. It is so largely unregulated that many of the schools don't even pass basic health and safety necessities required for pupils when they attend a school (because they aren't registered).
This makes it so much clearer as the school is still operating because the department of education may not know that it exists. I don't want to state this as a fact but i am close to 100% sure this is the case as there is absolutely no possible way that this place would pass health and safety check because it is falling apart.
I don't know the details of your visit to the building but i have a strong feeling you went at a time in which the school could have been on a holiday. It makes sense that the library is in such a good condition because that area is maintained by the pupils and staff of the school and i saw them enter and exit it many times. In essence what i know is that it is still a practising almost certainly an unregistered illegal school and i would advise that you do not go there as they were not impressed by our presence.