ramrod straight
Having a very erect posture, typically accompanied by a serious or formal demeanor. He sat ramrod straight at the table, glowering at me throughout the meal. She's always ramrod straight during class, studiously taking notes.
ˌramrod ˈstraight
, (as) ˌstraight as a ˈramrod
(of a person) with a very straight back and looking serious and formal: As she walked in she could feel the tension in the room, with her mother sitting ramrod straight in her chair.A ramrod is a long straight piece of iron used in the past to push explosive into a gun.