Sitting Posture for Violinists and Violists
Most orchestral playing is done sitting down. One general posture principle when sitting: students should be able to stand up without shifting their weight forward. Ask them to stand up when you give a signal, like snapping your fingers to assess if your students are balanced when sitting.
Suggestions for Improving Posture
Positive remarks and modeling work better than negative comments when trying to improve posture in the string class. Saying "don't slouch" to a student gives them no information about what you actually WANT them to do. Model an incorrect posture for students and get them to imitate you. Then model the transition to correct posture and ask them to do the same.
Young students will become tired if required to hold their instruments in playing position for too long. Build variety of standing and sitting into the routine. Try movement activities and stretching to build physical awareness of motion into the routine.
Kato Havas developed a "new approach" to release physical tension in string playing. Yehudi Menuhin was a strong advocate of using exercise and mental relaxation techniques. Helping your students develop physical awareness and relaxation anf freeing their bodies from tension will greatly reduce the chance of injury as they develop.