"Put your thinking caps on."
"No you don't go straight into medical if you hurt yourself, you tell a teacher first."
"Go and see if the other classes have a spare milk. Take Sarah with you." Yes, the free school milk. At that age we were always put in pairs for small errands or "messages" round the school.
"Don't run with scissors."
"Get into twos, and hold hands. Who's going to hold the doors today?" The route from the classroom to the assembly hall and playground had several swing doors; so lots of crowd control and lining up was needed getting children through these, several times a day. One boy with the duty of holding the door used to put his hand out and say "money!" I remember asking why there were all these doors; my teacher agreed that they were a nuisance. They had labels on them saying "fire door keep shut", but I thought that meant they shouldn't be opened at all.
When the fearsome deputy head condemned a child to missing playtime, she did it with one word: "IN!!!!!". Everyone knew what that meant.
"Are the girls going to beat the boys today?" Some teachers encouraged lots of competition between them.
"Boys will be noughts, girls will be crosses." Always that way round.
When we learned about Victorian schools, we occasionally heard phrases such as
"...or you'll be strung up to ceiling in a basket."
"The cane might be brought back one day, you know."
I never heard it used for real, but I liked the saying "procrastination is the thief of time". There was a story about a girl having to write this out repeatedly, and the teacher's reaction when she saw that "procrastination" had been spelled four different ways.