All teachers enjoy the general indemnity provided by their employer for their professional liability, but this does not protect them from the consequences of their own negligence. Thus, if an accident occurs in the classroom or laboratory and the teacher is held to have been negligent, there is no indemnity which would protect them from accepting their professional responsibilities simply on the grounds that there were too many children in the room.
Putting it the other way round, it is the duty of a teacher to ensure that whatever he does with the class is safe and reasonable in all the circumstances.
Many science and technology teachers are daily confronted with this problem and there is no simple answer which matches all possible situations. However, it may be helpful in some cases to point out that additional supervision may be provided within the laboratory or workshop by making use of technicians or even parent volunteers to create a better ratio of adults to pupils.