When the teachers drink Ethel's drink, they empty their glasses and ask for more. Ethel only pours more into Miss Cackle's glass yet in the next shot we see all three teachers drinking more juice.
Ruby and some of the other girls are apparently still away for some reason. They had previously gone bird watching with Miss Bat and Miss Cackle, who are now back at the school. So where are these girls and what teachers are supervising them?