I wonder if it is to avoid DBS (used to be CRB) checks which surely must be necessary with the word school in the title. Any convicted paedophile within the congregation would fail and therefore not be permitted to attend that meeting. I don't believe it is the cost of the check because there is no way the society would pay members would.
Whatever the reason it is was the most boring of all the meetings and you knew who had a part because with certain people it was the only time they turned up.
Our meetings in Spain were invariably shorter because there were not enough bros to take the parts and even though small congs were allowed to use sisters they never did.
Whatever the reason it will be to benefit the society the most or to inconvenience it the least in complying with secular authority.