So, just to be clear is the scenario something like this?
Year 8 Computing class. Sarah the student starts sobbing/ having a nosebleed, so teacher takes her outside to find out what's wrong. They chat outside for a few minutes. Sarah needs to go to Pastoral Office or nurse but as she is so distressed or I'll, teacher walks her down the corridor.
While teacher is out of the room, James stands up to go to the bin. As he walks towards it, he trips over Lucy's chair, falling and injuring his head.
If James hadn't fallen whilst the teacher was not present, the teacher is liable for nothing, as nothing happened. (If it came to light that a teacher habitually left the classroom they would probably be in breach of contract for not fulfilling adequate teaching and learning obligations.)
As James did fall, the investigation would consider whether the teacher did the most reasonable thing to preserve safety - was the room overcrowded, is Lucy known to be disruptive when unsupervised? So yes, the teacher could be held responsible but it would depend on the level of risk the class was left in, as perceived by the investigation.